OpenCloudOS-Kernel/fs/btrfs/scrub.c

/*
 * Copyright (C) 2011 STRATO.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/blkdev.h>
#include <linux/ratelimit.h>
#include "ctree.h"
#include "volumes.h"
#include "disk-io.h"
#include "ordered-data.h"
#include "transaction.h"
#include "backref.h"
#include "extent_io.h"
#include "check-integrity.h"

/*
 * This is only the first step towards a full-features scrub. It reads all
 * extent and super block and verifies the checksums. In case a bad checksum
 * is found or the extent cannot be read, good data will be written back if
 * any can be found.
 *
 * Future enhancements:
 *  - In case an unrepairable extent is encountered, track which files are
 *    affected and report them
 *  - track and record media errors, throw out bad devices
 *  - add a mode to also read unallocated space
 */

struct scrub_block;
struct scrub_dev;

#define SCRUB_PAGES_PER_BIO	16	/* 64k per bio */
#define SCRUB_BIOS_PER_DEV	16	/* 1 MB per device in flight */
#define SCRUB_MAX_PAGES_PER_BLOCK	16	/* 64k per node/leaf/sector */

struct scrub_page {
	struct scrub_block	*sblock;
	struct page		*page;
	struct btrfs_device	*dev;
	u64			flags;  /* extent flags */
	u64			generation;
	u64			logical;
	u64			physical;
	struct {
		unsigned int	mirror_num:8;
		unsigned int	have_csum:1;
		unsigned int	io_error:1;
	};
	u8			csum[BTRFS_CSUM_SIZE];
};

struct scrub_bio {
	int			index;
	struct scrub_dev	*sdev;
	struct bio		*bio;
	int			err;
	u64			logical;
	u64			physical;
	struct scrub_page	*pagev[SCRUB_PAGES_PER_BIO];
	int			page_count;
	int			next_free;
	struct btrfs_work	work;
};

struct scrub_block {
	struct scrub_page	pagev[SCRUB_MAX_PAGES_PER_BLOCK];
	int			page_count;
	atomic_t		outstanding_pages;
	atomic_t		ref_count; /* free mem on transition to zero */
	struct scrub_dev	*sdev;
	struct {
		unsigned int	header_error:1;
		unsigned int	checksum_error:1;
		unsigned int	no_io_error_seen:1;
		unsigned int	generation_error:1; /* also sets header_error */
	};
};

struct scrub_dev {
	struct scrub_bio	*bios[SCRUB_BIOS_PER_DEV];
	struct btrfs_device	*dev;
	int			first_free;
	int			curr;
	atomic_t		in_flight;
	atomic_t		fixup_cnt;
	spinlock_t		list_lock;
	wait_queue_head_t	list_wait;
	u16			csum_size;
	struct list_head	csum_list;
	atomic_t		cancel_req;
	int			readonly;
	int			pages_per_bio; /* <= SCRUB_PAGES_PER_BIO */
	u32			sectorsize;
	u32			nodesize;
	u32			leafsize;
	/*
	 * statistics
	 */
	struct btrfs_scrub_progress stat;
	spinlock_t		stat_lock;
};

struct scrub_fixup_nodatasum {
	struct scrub_dev	*sdev;
	u64			logical;
	struct btrfs_root	*root;
	struct btrfs_work	work;
	int			mirror_num;
};

struct scrub_warning {
	struct btrfs_path	*path;
	u64			extent_item_size;
	char			*scratch_buf;
	char			*msg_buf;
	const char		*errstr;
	sector_t		sector;
	u64			logical;
	struct btrfs_device	*dev;
	int			msg_bufsize;
	int			scratch_bufsize;
};


static int scrub_handle_errored_block(struct scrub_block *sblock_to_check);
static int scrub_setup_recheck_block(struct scrub_dev *sdev,
				     struct btrfs_mapping_tree *map_tree,
				     u64 length, u64 logical,
				     struct scrub_block *sblock);
static int scrub_recheck_block(struct btrfs_fs_info *fs_info,
			       struct scrub_block *sblock, int is_metadata,
			       int have_csum, u8 *csum, u64 generation,
			       u16 csum_size);
static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
					 struct scrub_block *sblock,
					 int is_metadata, int have_csum,
					 const u8 *csum, u64 generation,
					 u16 csum_size);
static void scrub_complete_bio_end_io(struct bio *bio, int err);
static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
					     struct scrub_block *sblock_good,
					     int force_write);
static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
					    struct scrub_block *sblock_good,
					    int page_num, int force_write);
static int scrub_checksum_data(struct scrub_block *sblock);
static int scrub_checksum_tree_block(struct scrub_block *sblock);
static int scrub_checksum_super(struct scrub_block *sblock);
static void scrub_block_get(struct scrub_block *sblock);
static void scrub_block_put(struct scrub_block *sblock);
static int scrub_add_page_to_bio(struct scrub_dev *sdev,
				 struct scrub_page *spage);
static int scrub_pages(struct scrub_dev *sdev, u64 logical, u64 len,
		       u64 physical, u64 flags, u64 gen, int mirror_num,
		       u8 *csum, int force);
static void scrub_bio_end_io(struct bio *bio, int err);
static void scrub_bio_end_io_worker(struct btrfs_work *work);
static void scrub_block_complete(struct scrub_block *sblock);


static void scrub_free_csums(struct scrub_dev *sdev)
{
	while (!list_empty(&sdev->csum_list)) {
		struct btrfs_ordered_sum *sum;
		sum = list_first_entry(&sdev->csum_list,
				       struct btrfs_ordered_sum, list);
		list_del(&sum->list);
		kfree(sum);
	}
}

static noinline_for_stack void scrub_free_dev(struct scrub_dev *sdev)
{
	int i;

	if (!sdev)
		return;

	/* this can happen when scrub is cancelled */
	if (sdev->curr != -1) {
		struct scrub_bio *sbio = sdev->bios[sdev->curr];

		for (i = 0; i < sbio->page_count; i++) {
			BUG_ON(!sbio->pagev[i]);
			BUG_ON(!sbio->pagev[i]->page);
			scrub_block_put(sbio->pagev[i]->sblock);
		}
		bio_put(sbio->bio);
	}

	for (i = 0; i < SCRUB_BIOS_PER_DEV; ++i) {
		struct scrub_bio *sbio = sdev->bios[i];

		if (!sbio)
			break;
		kfree(sbio);
	}

	scrub_free_csums(sdev);
	kfree(sdev);
}

static noinline_for_stack
struct scrub_dev *scrub_setup_dev(struct btrfs_device *dev)
{
	struct scrub_dev *sdev;
	int		i;
	struct btrfs_fs_info *fs_info = dev->dev_root->fs_info;
	int pages_per_bio;

	pages_per_bio = min_t(int, SCRUB_PAGES_PER_BIO,
			      bio_get_nr_vecs(dev->bdev));
	sdev = kzalloc(sizeof(*sdev), GFP_NOFS);
	if (!sdev)
		goto nomem;
	sdev->dev = dev;
	sdev->pages_per_bio = pages_per_bio;
	sdev->curr = -1;
	for (i = 0; i < SCRUB_BIOS_PER_DEV; ++i) {
		struct scrub_bio *sbio;

		sbio = kzalloc(sizeof(*sbio), GFP_NOFS);
		if (!sbio)
			goto nomem;
		sdev->bios[i] = sbio;

		sbio->index = i;
		sbio->sdev = sdev;
		sbio->page_count = 0;
		sbio->work.func = scrub_bio_end_io_worker;

		if (i != SCRUB_BIOS_PER_DEV-1)
			sdev->bios[i]->next_free = i + 1;
		else
			sdev->bios[i]->next_free = -1;
	}
	sdev->first_free = 0;
	sdev->nodesize = dev->dev_root->nodesize;
	sdev->leafsize = dev->dev_root->leafsize;
	sdev->sectorsize = dev->dev_root->sectorsize;
	atomic_set(&sdev->in_flight, 0);
	atomic_set(&sdev->fixup_cnt, 0);
	atomic_set(&sdev->cancel_req, 0);
	sdev->csum_size = btrfs_super_csum_size(fs_info->super_copy);
	INIT_LIST_HEAD(&sdev->csum_list);

	spin_lock_init(&sdev->list_lock);
	spin_lock_init(&sdev->stat_lock);
	init_waitqueue_head(&sdev->list_wait);
	return sdev;

nomem:
	scrub_free_dev(sdev);
	return ERR_PTR(-ENOMEM);
}

static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root, void *ctx)
{
	u64 isize;
	u32 nlink;
	int ret;
	int i;
	struct extent_buffer *eb;
	struct btrfs_inode_item *inode_item;
	struct scrub_warning *swarn = ctx;
	struct btrfs_fs_info *fs_info = swarn->dev->dev_root->fs_info;
	struct inode_fs_paths *ipath = NULL;
	struct btrfs_root *local_root;
	struct btrfs_key root_key;

	root_key.objectid = root;
	root_key.type = BTRFS_ROOT_ITEM_KEY;
	root_key.offset = (u64)-1;
	local_root = btrfs_read_fs_root_no_name(fs_info, &root_key);
	if (IS_ERR(local_root)) {
		ret = PTR_ERR(local_root);
		goto err;
	}

	ret = inode_item_info(inum, 0, local_root, swarn->path);
	if (ret) {
		btrfs_release_path(swarn->path);
		goto err;
	}

	eb = swarn->path->nodes[0];
	inode_item = btrfs_item_ptr(eb, swarn->path->slots[0],
					struct btrfs_inode_item);
	isize = btrfs_inode_size(eb, inode_item);
	nlink = btrfs_inode_nlink(eb, inode_item);
	btrfs_release_path(swarn->path);

	ipath = init_ipath(4096, local_root, swarn->path);
	if (IS_ERR(ipath)) {
		ret = PTR_ERR(ipath);
		ipath = NULL;
		goto err;
	}
	ret = paths_from_inode(inum, ipath);

	if (ret < 0)
		goto err;

	/*
	 * we deliberately ignore the bit ipath might have been too small to
	 * hold all of the paths here
	 */
	for (i = 0; i < ipath->fspath->elem_cnt; ++i)
		printk(KERN_WARNING "btrfs: %s at logical %llu on dev "
			"%s, sector %llu, root %llu, inode %llu, offset %llu, "
			"length %llu, links %u (path: %s)\n", swarn->errstr,
			swarn->logical, swarn->dev->name,
			(unsigned long long)swarn->sector, root, inum, offset,
			min(isize - offset, (u64)PAGE_SIZE), nlink,
			(char *)(unsigned long)ipath->fspath->val[i]);

	free_ipath(ipath);
	return 0;

err:
	printk(KERN_WARNING "btrfs: %s at logical %llu on dev "
		"%s, sector %llu, root %llu, inode %llu, offset %llu: path "
		"resolving failed with ret=%d\n", swarn->errstr,
		swarn->logical, swarn->dev->name,
		(unsigned long long)swarn->sector, root, inum, offset, ret);

	free_ipath(ipath);
	return 0;
}

static void scrub_print_warning(const char *errstr, struct scrub_block *sblock)
{
	struct btrfs_device *dev = sblock->sdev->dev;
	struct btrfs_fs_info *fs_info = dev->dev_root->fs_info;
	struct btrfs_path *path;
	struct btrfs_key found_key;
	struct extent_buffer *eb;
	struct btrfs_extent_item *ei;
	struct scrub_warning swarn;
	u32 item_size;
	int ret;
	u64 ref_root;
	u8 ref_level;
	unsigned long ptr = 0;
	const int bufsize = 4096;
	u64 extent_item_pos;

	path = btrfs_alloc_path();

	swarn.scratch_buf = kmalloc(bufsize, GFP_NOFS);
	swarn.msg_buf = kmalloc(bufsize, GFP_NOFS);
	BUG_ON(sblock->page_count < 1);
	swarn.sector = (sblock->pagev[0].physical) >> 9;
	swarn.logical = sblock->pagev[0].logical;
	swarn.errstr = errstr;
	swarn.dev = dev;
	swarn.msg_bufsize = bufsize;
	swarn.scratch_bufsize = bufsize;

	if (!path || !swarn.scratch_buf || !swarn.msg_buf)
		goto out;

	ret = extent_from_logical(fs_info, swarn.logical, path, &found_key);
	if (ret < 0)
		goto out;

	extent_item_pos = swarn.logical - found_key.objectid;
	swarn.extent_item_size = found_key.offset;

	eb = path->nodes[0];
	ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
	item_size = btrfs_item_size_nr(eb, path->slots[0]);
	btrfs_release_path(path);

	if (ret & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
		do {
			ret = tree_backref_for_extent(&ptr, eb, ei, item_size,
							&ref_root, &ref_level);
			printk(KERN_WARNING
				"btrfs: %s at logical %llu on dev %s, "
				"sector %llu: metadata %s (level %d) in tree "
				"%llu\n", errstr, swarn.logical, dev->name,
				(unsigned long long)swarn.sector,
				ref_level ? "node" : "leaf",
				ret < 0 ? -1 : ref_level,
				ret < 0 ? -1 : ref_root);
		} while (ret != 1);
	} else {
		swarn.path = path;
		iterate_extent_inodes(fs_info, found_key.objectid,
					extent_item_pos, 1,
					scrub_print_warning_inode, &swarn);
	}

out:
	btrfs_free_path(path);
	kfree(swarn.scratch_buf);
	kfree(swarn.msg_buf);
}

static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *ctx)
{
	struct page *page = NULL;
	unsigned long index;
	struct scrub_fixup_nodatasum *fixup = ctx;
	int ret;
	int corrected = 0;
	struct btrfs_key key;
	struct inode *inode = NULL;
	u64 end = offset + PAGE_SIZE - 1;
	struct btrfs_root *local_root;

	key.objectid = root;
	key.type = BTRFS_ROOT_ITEM_KEY;
	key.offset = (u64)-1;
	local_root = btrfs_read_fs_root_no_name(fixup->root->fs_info, &key);
	if (IS_ERR(local_root))
		return PTR_ERR(local_root);

	key.type = BTRFS_INODE_ITEM_KEY;
	key.objectid = inum;
	key.offset = 0;
	inode = btrfs_iget(fixup->root->fs_info->sb, &key, local_root, NULL);
	if (IS_ERR(inode))
		return PTR_ERR(inode);

	index = offset >> PAGE_CACHE_SHIFT;

	page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
	if (!page) {
		ret = -ENOMEM;
		goto out;
	}

	if (PageUptodate(page)) {
		struct btrfs_mapping_tree *map_tree;
		if (PageDirty(page)) {
			/*
			 * we need to write the data to the defect sector. the
			 * data that was in that sector is not in memory,
			 * because the page was modified. we must not write the
			 * modified page to that sector.
			 *
			 * TODO: what could be done here: wait for the delalloc
			 *       runner to write out that page (might involve
			 *       COW) and see whether the sector is still
			 *       referenced afterwards.
			 *
			 * For the meantime, we'll treat this error
			 * incorrectable, although there is a chance that a
			 * later scrub will find the bad sector again and that
			 * there's no dirty page in memory, then.
			 */
			ret = -EIO;
			goto out;
		}
		map_tree = &BTRFS_I(inode)->root->fs_info->mapping_tree;
		ret = repair_io_failure(map_tree, offset, PAGE_SIZE,
					fixup->logical, page,
					fixup->mirror_num);
		unlock_page(page);
		corrected = !ret;
	} else {
		/*
		 * we need to get good data first. the general readpage path
		 * will call repair_io_failure for us, we just have to make
		 * sure we read the bad mirror.
		 */
		ret = set_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
					EXTENT_DAMAGED, GFP_NOFS);
		if (ret) {
			/* set_extent_bits should give proper error */
			WARN_ON(ret > 0);
			if (ret > 0)
				ret = -EFAULT;
			goto out;
		}

		ret = extent_read_full_page(&BTRFS_I(inode)->io_tree, page,
						btrfs_get_extent,
						fixup->mirror_num);
		wait_on_page_locked(page);

		corrected = !test_range_bit(&BTRFS_I(inode)->io_tree, offset,
						end, EXTENT_DAMAGED, 0, NULL);
		if (!corrected)
			clear_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
						EXTENT_DAMAGED, GFP_NOFS);
	}

out:
	if (page)
		put_page(page);
	if (inode)
		iput(inode);

	if (ret < 0)
		return ret;

	if (ret == 0 && corrected) {
		/*
		 * we only need to call readpage for one of the inodes belonging
		 * to this extent. so make iterate_extent_inodes stop
		 */
		return 1;
	}

	return -EIO;
}

static void scrub_fixup_nodatasum(struct btrfs_work *work)
{
	int ret;
	struct scrub_fixup_nodatasum *fixup;
	struct scrub_dev *sdev;
	struct btrfs_trans_handle *trans = NULL;
	struct btrfs_fs_info *fs_info;
	struct btrfs_path *path;
	int uncorrectable = 0;

	fixup = container_of(work, struct scrub_fixup_nodatasum, work);
	sdev = fixup->sdev;
	fs_info = fixup->root->fs_info;

	path = btrfs_alloc_path();
	if (!path) {
		spin_lock(&sdev->stat_lock);
		++sdev->stat.malloc_errors;
		spin_unlock(&sdev->stat_lock);
		uncorrectable = 1;
		goto out;
	}

	trans = btrfs_join_transaction(fixup->root);
	if (IS_ERR(trans)) {
		uncorrectable = 1;
		goto out;
	}

	/*
	 * the idea is to trigger a regular read through the standard path. we
	 * read a page from the (failed) logical address by specifying the
	 * corresponding copynum of the failed sector. thus, that readpage is
	 * expected to fail.
	 * that is the point where on-the-fly error correction will kick in
	 * (once it's finished) and rewrite the failed sector if a good copy
	 * can be found.
	 */
	ret = iterate_inodes_from_logical(fixup->logical, fixup->root->fs_info,
						path, scrub_fixup_readpage,
						fixup);
	if (ret < 0) {
		uncorrectable = 1;
		goto out;
	}
	WARN_ON(ret != 1);

	spin_lock(&sdev->stat_lock);
	++sdev->stat.corrected_errors;
	spin_unlock(&sdev->stat_lock);

out:
	if (trans && !IS_ERR(trans))
		btrfs_end_transaction(trans, fixup->root);
	if (uncorrectable) {
		spin_lock(&sdev->stat_lock);
		++sdev->stat.uncorrectable_errors;
		spin_unlock(&sdev->stat_lock);
		printk_ratelimited(KERN_ERR
			"btrfs: unable to fixup (nodatasum) error at logical %llu on dev %s\n",
			(unsigned long long)fixup->logical, sdev->dev->name);
	}

	btrfs_free_path(path);
	kfree(fixup);

	/* see caller why we're pretending to be paused in the scrub counters */
	mutex_lock(&fs_info->scrub_lock);
	atomic_dec(&fs_info->scrubs_running);
	atomic_dec(&fs_info->scrubs_paused);
	mutex_unlock(&fs_info->scrub_lock);
	atomic_dec(&sdev->fixup_cnt);
	wake_up(&fs_info->scrub_pause_wait);
	wake_up(&sdev->list_wait);
}

/*
 * scrub_handle_errored_block gets called when either verification of the
 * pages failed or the bio failed to read, e.g. with EIO. In the latter
 * case, this function handles all pages in the bio, even though only one
 * may be bad.
 * The goal of this function is to repair the errored block by using the
 * contents of one of the mirrors.
 */
static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
{
	struct scrub_dev *sdev = sblock_to_check->sdev;
	struct btrfs_fs_info *fs_info;
	u64 length;
	u64 logical;
	u64 generation;
	unsigned int failed_mirror_index;
	unsigned int is_metadata;
	unsigned int have_csum;
	u8 *csum;
	struct scrub_block *sblocks_for_recheck; /* holds one for each mirror */
	struct scrub_block *sblock_bad;
	int ret;
	int mirror_index;
	int page_num;
	int success;
	static DEFINE_RATELIMIT_STATE(_rs, DEFAULT_RATELIMIT_INTERVAL,
				      DEFAULT_RATELIMIT_BURST);

	BUG_ON(sblock_to_check->page_count < 1);
	fs_info = sdev->dev->dev_root->fs_info;
	length = sblock_to_check->page_count * PAGE_SIZE;
	logical = sblock_to_check->pagev[0].logical;
	generation = sblock_to_check->pagev[0].generation;
	BUG_ON(sblock_to_check->pagev[0].mirror_num < 1);
	failed_mirror_index = sblock_to_check->pagev[0].mirror_num - 1;
	is_metadata = !(sblock_to_check->pagev[0].flags &
			BTRFS_EXTENT_FLAG_DATA);
	have_csum = sblock_to_check->pagev[0].have_csum;
	csum = sblock_to_check->pagev[0].csum;

	/*
	 * read all mirrors one after the other. This includes to
	 * re-read the extent or metadata block that failed (that was
	 * the cause that this fixup code is called) another time,
	 * page by page this time in order to know which pages
	 * caused I/O errors and which ones are good (for all mirrors).
	 * It is the goal to handle the situation when more than one
	 * mirror contains I/O errors, but the errors do not
	 * overlap, i.e. the data can be repaired by selecting the
	 * pages from those mirrors without I/O error on the
	 * particular pages. One example (with blocks >= 2 * PAGE_SIZE)
	 * would be that mirror #1 has an I/O error on the first page,
	 * the second page is good, and mirror #2 has an I/O error on
	 * the second page, but the first page is good.
	 * Then the first page of the first mirror can be repaired by
	 * taking the first page of the second mirror, and the
	 * second page of the second mirror can be repaired by
	 * copying the contents of the 2nd page of the 1st mirror.
	 * One more note: if the pages of one mirror contain I/O
	 * errors, the checksum cannot be verified. In order to get
	 * the best data for repairing, the first attempt is to find
	 * a mirror without I/O errors and with a validated checksum.
	 * Only if this is not possible, the pages are picked from
	 * mirrors with I/O errors without considering the checksum.
	 * If the latter is the case, at the end, the checksum of the
	 * repaired area is verified in order to correctly maintain
	 * the statistics.
	 */

	sblocks_for_recheck = kzalloc(BTRFS_MAX_MIRRORS *
				     sizeof(*sblocks_for_recheck),
				     GFP_NOFS);
	if (!sblocks_for_recheck) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.malloc_errors++;
		sdev->stat.read_errors++;
		sdev->stat.uncorrectable_errors++;
		spin_unlock(&sdev->stat_lock);
		btrfs_dev_stat_inc_and_print(sdev->dev,
					     BTRFS_DEV_STAT_READ_ERRS);
		goto out;
	}

	/* setup the context, map the logical blocks and alloc the pages */
	ret = scrub_setup_recheck_block(sdev, &fs_info->mapping_tree, length,
					logical, sblocks_for_recheck);
	if (ret) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.read_errors++;
		sdev->stat.uncorrectable_errors++;
		spin_unlock(&sdev->stat_lock);
		btrfs_dev_stat_inc_and_print(sdev->dev,
					     BTRFS_DEV_STAT_READ_ERRS);
		goto out;
	}
	BUG_ON(failed_mirror_index >= BTRFS_MAX_MIRRORS);
	sblock_bad = sblocks_for_recheck + failed_mirror_index;

	/* build and submit the bios for the failed mirror, check checksums */
	ret = scrub_recheck_block(fs_info, sblock_bad, is_metadata, have_csum,
				  csum, generation, sdev->csum_size);
	if (ret) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.read_errors++;
		sdev->stat.uncorrectable_errors++;
		spin_unlock(&sdev->stat_lock);
		btrfs_dev_stat_inc_and_print(sdev->dev,
					     BTRFS_DEV_STAT_READ_ERRS);
		goto out;
	}

	if (!sblock_bad->header_error && !sblock_bad->checksum_error &&
	    sblock_bad->no_io_error_seen) {
		/*
		 * the error disappeared after reading page by page, or
		 * the area was part of a huge bio and other parts of the
		 * bio caused I/O errors, or the block layer merged several
		 * read requests into one and the error is caused by a
		 * different bio (usually one of the two latter cases is
		 * the cause)
		 */
		spin_lock(&sdev->stat_lock);
		sdev->stat.unverified_errors++;
		spin_unlock(&sdev->stat_lock);

		goto out;
	}

	if (!sblock_bad->no_io_error_seen) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.read_errors++;
		spin_unlock(&sdev->stat_lock);
		if (__ratelimit(&_rs))
			scrub_print_warning("i/o error", sblock_to_check);
		btrfs_dev_stat_inc_and_print(sdev->dev,
					     BTRFS_DEV_STAT_READ_ERRS);
	} else if (sblock_bad->checksum_error) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.csum_errors++;
		spin_unlock(&sdev->stat_lock);
		if (__ratelimit(&_rs))
			scrub_print_warning("checksum error", sblock_to_check);
		btrfs_dev_stat_inc_and_print(sdev->dev,
					     BTRFS_DEV_STAT_CORRUPTION_ERRS);
	} else if (sblock_bad->header_error) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.verify_errors++;
		spin_unlock(&sdev->stat_lock);
		if (__ratelimit(&_rs))
			scrub_print_warning("checksum/header error",
					    sblock_to_check);
		if (sblock_bad->generation_error)
			btrfs_dev_stat_inc_and_print(sdev->dev,
				BTRFS_DEV_STAT_GENERATION_ERRS);
		else
			btrfs_dev_stat_inc_and_print(sdev->dev,
				BTRFS_DEV_STAT_CORRUPTION_ERRS);
	}

	if (sdev->readonly)
		goto did_not_correct_error;

	if (!is_metadata && !have_csum) {
		struct scrub_fixup_nodatasum *fixup_nodatasum;

		/*
		 * !is_metadata and !have_csum, this means that the data
		 * might not be COW'ed, that it might be modified
		 * concurrently. The general strategy to work on the
		 * commit root does not help in the case when COW is not
		 * used.
		 */
		fixup_nodatasum = kzalloc(sizeof(*fixup_nodatasum), GFP_NOFS);
		if (!fixup_nodatasum)
			goto did_not_correct_error;
		fixup_nodatasum->sdev = sdev;
		fixup_nodatasum->logical = logical;
		fixup_nodatasum->root = fs_info->extent_root;
		fixup_nodatasum->mirror_num = failed_mirror_index + 1;
		/*
		 * increment scrubs_running to prevent cancel requests from
		 * completing as long as a fixup worker is running. we must also
		 * increment scrubs_paused to prevent deadlocking on pause
		 * requests used for transactions commits (as the worker uses a
		 * transaction context). it is safe to regard the fixup worker
		 * as paused for all matters practical. effectively, we only
		 * avoid cancellation requests from completing.
		 */
		mutex_lock(&fs_info->scrub_lock);
		atomic_inc(&fs_info->scrubs_running);
		atomic_inc(&fs_info->scrubs_paused);
		mutex_unlock(&fs_info->scrub_lock);
		atomic_inc(&sdev->fixup_cnt);
		fixup_nodatasum->work.func = scrub_fixup_nodatasum;
		btrfs_queue_worker(&fs_info->scrub_workers,
				   &fixup_nodatasum->work);
		goto out;
	}

	/*
	 * now build and submit the bios for the other mirrors, check
	 * checksums
	 */
	for (mirror_index = 0;
	     mirror_index < BTRFS_MAX_MIRRORS &&
	     sblocks_for_recheck[mirror_index].page_count > 0;
	     mirror_index++) {
		if (mirror_index == failed_mirror_index)
			continue;

		/* build and submit the bios, check checksums */
		ret = scrub_recheck_block(fs_info,
					  sblocks_for_recheck + mirror_index,
					  is_metadata, have_csum, csum,
					  generation, sdev->csum_size);
		if (ret)
			goto did_not_correct_error;
	}

	/*
	 * first try to pick the mirror which is completely without I/O
	 * errors and also does not have a checksum error.
	 * If one is found, and if a checksum is present, the full block
	 * that is known to contain an error is rewritten. Afterwards
	 * the block is known to be corrected.
	 * If a mirror is found which is completely correct, and no
	 * checksum is present, only those pages are rewritten that had
	 * an I/O error in the block to be repaired, since it cannot be
	 * determined, which copy of the other pages is better (and it
	 * could happen otherwise that a correct page would be
	 * overwritten by a bad one).
	 */
	for (mirror_index = 0;
	     mirror_index < BTRFS_MAX_MIRRORS &&
	     sblocks_for_recheck[mirror_index].page_count > 0;
	     mirror_index++) {
		struct scrub_block *sblock_other = sblocks_for_recheck +
						   mirror_index;

		if (!sblock_other->header_error &&
		    !sblock_other->checksum_error &&
		    sblock_other->no_io_error_seen) {
			int force_write = is_metadata || have_csum;

			ret = scrub_repair_block_from_good_copy(sblock_bad,
								sblock_other,
								force_write);
			if (0 == ret)
				goto corrected_error;
		}
	}

	/*
	 * in case of I/O errors in the area that is supposed to be
	 * repaired, continue by picking good copies of those pages.
	 * Select the good pages from mirrors to rewrite bad pages from
	 * the area to fix. Afterwards verify the checksum of the block
	 * that is supposed to be repaired. This verification step is
	 * only done for the purpose of statistic counting and for the
	 * final scrub report, whether errors remain.
	 * A perfect algorithm could make use of the checksum and try
	 * all possible combinations of pages from the different mirrors
	 * until the checksum verification succeeds. For example, when
	 * the 2nd page of mirror #1 faces I/O errors, and the 2nd page
	 * of mirror #2 is readable but the final checksum test fails,
	 * then the 2nd page of mirror #3 could be tried, whether now
	 * the final checksum succeedes. But this would be a rare
	 * exception and is therefore not implemented. At least it is
	 * avoided that the good copy is overwritten.
	 * A more useful improvement would be to pick the sectors
	 * without I/O error based on sector sizes (512 bytes on legacy
	 * disks) instead of on PAGE_SIZE. Then maybe 512 byte of one
	 * mirror could be repaired by taking 512 byte of a different
	 * mirror, even if other 512 byte sectors in the same PAGE_SIZE
	 * area are unreadable.
	 */

	/* can only fix I/O errors from here on */
	if (sblock_bad->no_io_error_seen)
		goto did_not_correct_error;

	success = 1;
	for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
		struct scrub_page *page_bad = sblock_bad->pagev + page_num;

		if (!page_bad->io_error)
			continue;

		for (mirror_index = 0;
		     mirror_index < BTRFS_MAX_MIRRORS &&
		     sblocks_for_recheck[mirror_index].page_count > 0;
		     mirror_index++) {
			struct scrub_block *sblock_other = sblocks_for_recheck +
							   mirror_index;
			struct scrub_page *page_other = sblock_other->pagev +
							page_num;

			if (!page_other->io_error) {
				ret = scrub_repair_page_from_good_copy(
					sblock_bad, sblock_other, page_num, 0);
				if (0 == ret) {
					page_bad->io_error = 0;
					break; /* succeeded for this page */
				}
			}
		}

		if (page_bad->io_error) {
			/* did not find a mirror to copy the page from */
			success = 0;
		}
	}

	if (success) {
		if (is_metadata || have_csum) {
			/*
			 * need to verify the checksum now that all
			 * sectors on disk are repaired (the write
			 * request for data to be repaired is on its way).
			 * Just be lazy and use scrub_recheck_block()
			 * which re-reads the data before the checksum
			 * is verified, but most likely the data comes out
			 * of the page cache.
			 */
			ret = scrub_recheck_block(fs_info, sblock_bad,
						  is_metadata, have_csum, csum,
						  generation, sdev->csum_size);
			if (!ret && !sblock_bad->header_error &&
			    !sblock_bad->checksum_error &&
			    sblock_bad->no_io_error_seen)
				goto corrected_error;
			else
				goto did_not_correct_error;
		} else {
corrected_error:
			spin_lock(&sdev->stat_lock);
			sdev->stat.corrected_errors++;
			spin_unlock(&sdev->stat_lock);
			printk_ratelimited(KERN_ERR
				"btrfs: fixed up error at logical %llu on dev %s\n",
				(unsigned long long)logical, sdev->dev->name);
		}
	} else {
did_not_correct_error:
		spin_lock(&sdev->stat_lock);
		sdev->stat.uncorrectable_errors++;
		spin_unlock(&sdev->stat_lock);
		printk_ratelimited(KERN_ERR
			"btrfs: unable to fixup (regular) error at logical %llu on dev %s\n",
			(unsigned long long)logical, sdev->dev->name);
	}

out:
	if (sblocks_for_recheck) {
		for (mirror_index = 0; mirror_index < BTRFS_MAX_MIRRORS;
		     mirror_index++) {
			struct scrub_block *sblock = sblocks_for_recheck +
						     mirror_index;
			int page_index;

			for (page_index = 0; page_index < SCRUB_PAGES_PER_BIO;
			     page_index++)
				if (sblock->pagev[page_index].page)
					__free_page(
						sblock->pagev[page_index].page);
		}
		kfree(sblocks_for_recheck);
	}

	return 0;
}

static int scrub_setup_recheck_block(struct scrub_dev *sdev,
				     struct btrfs_mapping_tree *map_tree,
				     u64 length, u64 logical,
				     struct scrub_block *sblocks_for_recheck)
{
	int page_index;
	int mirror_index;
	int ret;

	/*
	 * note: the three members sdev, ref_count and outstanding_pages
	 * are not used (and not set) in the blocks that are used for
	 * the recheck procedure
	 */

	page_index = 0;
	while (length > 0) {
		u64 sublen = min_t(u64, length, PAGE_SIZE);
		u64 mapped_length = sublen;
		struct btrfs_bio *bbio = NULL;

		/*
		 * with a length of PAGE_SIZE, each returned stripe
		 * represents one mirror
		 */
		ret = btrfs_map_block(map_tree, WRITE, logical, &mapped_length,
				      &bbio, 0);
		if (ret || !bbio || mapped_length < sublen) {
			kfree(bbio);
			return -EIO;
		}

		BUG_ON(page_index >= SCRUB_PAGES_PER_BIO);
		for (mirror_index = 0; mirror_index < (int)bbio->num_stripes;
		     mirror_index++) {
			struct scrub_block *sblock;
			struct scrub_page *page;

			if (mirror_index >= BTRFS_MAX_MIRRORS)
				continue;

			sblock = sblocks_for_recheck + mirror_index;
			page = sblock->pagev + page_index;
			page->logical = logical;
			page->physical = bbio->stripes[mirror_index].physical;
			/* for missing devices, dev->bdev is NULL */
			page->dev = bbio->stripes[mirror_index].dev;
			page->mirror_num = mirror_index + 1;
			page->page = alloc_page(GFP_NOFS);
			if (!page->page) {
				spin_lock(&sdev->stat_lock);
				sdev->stat.malloc_errors++;
				spin_unlock(&sdev->stat_lock);
				return -ENOMEM;
			}
			sblock->page_count++;
		}
		kfree(bbio);
		length -= sublen;
		logical += sublen;
		page_index++;
	}

	return 0;
}

/*
 * this function will check the on disk data for checksum errors, header
 * errors and read I/O errors. If any I/O errors happen, the exact pages
 * which are errored are marked as being bad. The goal is to enable scrub
 * to take those pages that are not errored from all the mirrors so that
 * the pages that are errored in the just handled mirror can be repaired.
 */
static int scrub_recheck_block(struct btrfs_fs_info *fs_info,
			       struct scrub_block *sblock, int is_metadata,
			       int have_csum, u8 *csum, u64 generation,
			       u16 csum_size)
{
	int page_num;

	sblock->no_io_error_seen = 1;
	sblock->header_error = 0;
	sblock->checksum_error = 0;

	for (page_num = 0; page_num < sblock->page_count; page_num++) {
		struct bio *bio;
		int ret;
		struct scrub_page *page = sblock->pagev + page_num;
		DECLARE_COMPLETION_ONSTACK(complete);

		if (page->dev->bdev == NULL) {
			page->io_error = 1;
			sblock->no_io_error_seen = 0;
			continue;
		}

		BUG_ON(!page->page);
		bio = bio_alloc(GFP_NOFS, 1);
		if (!bio)
			return -EIO;
		bio->bi_bdev = page->dev->bdev;
		bio->bi_sector = page->physical >> 9;
		bio->bi_end_io = scrub_complete_bio_end_io;
		bio->bi_private = &complete;

		ret = bio_add_page(bio, page->page, PAGE_SIZE, 0);
		if (PAGE_SIZE != ret) {
			bio_put(bio);
			return -EIO;
		}
		btrfsic_submit_bio(READ, bio);

		/* this will also unplug the queue */
		wait_for_completion(&complete);

		page->io_error = !test_bit(BIO_UPTODATE, &bio->bi_flags);
		if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
			sblock->no_io_error_seen = 0;
		bio_put(bio);
	}

	if (sblock->no_io_error_seen)
		scrub_recheck_block_checksum(fs_info, sblock, is_metadata,
					     have_csum, csum, generation,
					     csum_size);

	return 0;
}

static void scrub_recheck_block_checksum(struct btrfs_fs_info *fs_info,
					 struct scrub_block *sblock,
					 int is_metadata, int have_csum,
					 const u8 *csum, u64 generation,
					 u16 csum_size)
{
	int page_num;
	u8 calculated_csum[BTRFS_CSUM_SIZE];
	u32 crc = ~(u32)0;
	struct btrfs_root *root = fs_info->extent_root;
	void *mapped_buffer;

	BUG_ON(!sblock->pagev[0].page);
	if (is_metadata) {
		struct btrfs_header *h;

		mapped_buffer = kmap_atomic(sblock->pagev[0].page);
		h = (struct btrfs_header *)mapped_buffer;

		if (sblock->pagev[0].logical != le64_to_cpu(h->bytenr) ||
		    memcmp(h->fsid, fs_info->fsid, BTRFS_UUID_SIZE) ||
		    memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
			   BTRFS_UUID_SIZE)) {
			sblock->header_error = 1;
		} else if (generation != le64_to_cpu(h->generation)) {
			sblock->header_error = 1;
			sblock->generation_error = 1;
		}
		csum = h->csum;
	} else {
		if (!have_csum)
			return;

		mapped_buffer = kmap_atomic(sblock->pagev[0].page);
	}

	for (page_num = 0;;) {
		if (page_num == 0 && is_metadata)
			crc = btrfs_csum_data(root,
				((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE,
				crc, PAGE_SIZE - BTRFS_CSUM_SIZE);
		else
			crc = btrfs_csum_data(root, mapped_buffer, crc,
					      PAGE_SIZE);

		kunmap_atomic(mapped_buffer);
		page_num++;
		if (page_num >= sblock->page_count)
			break;
		BUG_ON(!sblock->pagev[page_num].page);

		mapped_buffer = kmap_atomic(sblock->pagev[page_num].page);
	}

	btrfs_csum_final(crc, calculated_csum);
	if (memcmp(calculated_csum, csum, csum_size))
		sblock->checksum_error = 1;
}

static void scrub_complete_bio_end_io(struct bio *bio, int err)
{
	complete((struct completion *)bio->bi_private);
}

static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
					     struct scrub_block *sblock_good,
					     int force_write)
{
	int page_num;
	int ret = 0;

	for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
		int ret_sub;

		ret_sub = scrub_repair_page_from_good_copy(sblock_bad,
							   sblock_good,
							   page_num,
							   force_write);
		if (ret_sub)
			ret = ret_sub;
	}

	return ret;
}

static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
					    struct scrub_block *sblock_good,
					    int page_num, int force_write)
{
	struct scrub_page *page_bad = sblock_bad->pagev + page_num;
	struct scrub_page *page_good = sblock_good->pagev + page_num;

	BUG_ON(sblock_bad->pagev[page_num].page == NULL);
	BUG_ON(sblock_good->pagev[page_num].page == NULL);
	if (force_write || sblock_bad->header_error ||
	    sblock_bad->checksum_error || page_bad->io_error) {
		struct bio *bio;
		int ret;
		DECLARE_COMPLETION_ONSTACK(complete);

		bio = bio_alloc(GFP_NOFS, 1);
		if (!bio)
			return -EIO;
		bio->bi_bdev = page_bad->dev->bdev;
		bio->bi_sector = page_bad->physical >> 9;
		bio->bi_end_io = scrub_complete_bio_end_io;
		bio->bi_private = &complete;

		ret = bio_add_page(bio, page_good->page, PAGE_SIZE, 0);
		if (PAGE_SIZE != ret) {
			bio_put(bio);
			return -EIO;
		}
		btrfsic_submit_bio(WRITE, bio);

		/* this will also unplug the queue */
		wait_for_completion(&complete);
		if (!bio_flagged(bio, BIO_UPTODATE)) {
			btrfs_dev_stat_inc_and_print(page_bad->dev,
				BTRFS_DEV_STAT_WRITE_ERRS);
			bio_put(bio);
			return -EIO;
		}
		bio_put(bio);
	}

	return 0;
}

static void scrub_checksum(struct scrub_block *sblock)
{
	u64 flags;
	int ret;

	BUG_ON(sblock->page_count < 1);
	flags = sblock->pagev[0].flags;
	ret = 0;
	if (flags & BTRFS_EXTENT_FLAG_DATA)
		ret = scrub_checksum_data(sblock);
	else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
		ret = scrub_checksum_tree_block(sblock);
	else if (flags & BTRFS_EXTENT_FLAG_SUPER)
		(void)scrub_checksum_super(sblock);
	else
		WARN_ON(1);
	if (ret)
		scrub_handle_errored_block(sblock);
}

static int scrub_checksum_data(struct scrub_block *sblock)
{
	struct scrub_dev *sdev = sblock->sdev;
	u8 csum[BTRFS_CSUM_SIZE];
	u8 *on_disk_csum;
	struct page *page;
	void *buffer;
	u32 crc = ~(u32)0;
	int fail = 0;
	struct btrfs_root *root = sdev->dev->dev_root;
	u64 len;
	int index;

	BUG_ON(sblock->page_count < 1);
	if (!sblock->pagev[0].have_csum)
		return 0;

	on_disk_csum = sblock->pagev[0].csum;
	page = sblock->pagev[0].page;
	buffer = kmap_atomic(page);

	len = sdev->sectorsize;
	index = 0;
	for (;;) {
		u64 l = min_t(u64, len, PAGE_SIZE);

		crc = btrfs_csum_data(root, buffer, crc, l);
		kunmap_atomic(buffer);
		len -= l;
		if (len == 0)
			break;
		index++;
		BUG_ON(index >= sblock->page_count);
		BUG_ON(!sblock->pagev[index].page);
		page = sblock->pagev[index].page;
		buffer = kmap_atomic(page);
	}

	btrfs_csum_final(crc, csum);
	if (memcmp(csum, on_disk_csum, sdev->csum_size))
		fail = 1;

	return fail;
}

static int scrub_checksum_tree_block(struct scrub_block *sblock)
{
	struct scrub_dev *sdev = sblock->sdev;
	struct btrfs_header *h;
	struct btrfs_root *root = sdev->dev->dev_root;
	struct btrfs_fs_info *fs_info = root->fs_info;
	u8 calculated_csum[BTRFS_CSUM_SIZE];
	u8 on_disk_csum[BTRFS_CSUM_SIZE];
	struct page *page;
	void *mapped_buffer;
	u64 mapped_size;
	void *p;
	u32 crc = ~(u32)0;
	int fail = 0;
	int crc_fail = 0;
	u64 len;
	int index;

	BUG_ON(sblock->page_count < 1);
	page = sblock->pagev[0].page;
	mapped_buffer = kmap_atomic(page);
	h = (struct btrfs_header *)mapped_buffer;
	memcpy(on_disk_csum, h->csum, sdev->csum_size);

	/*
	 * we don't use the getter functions here, as we
	 * a) don't have an extent buffer and
	 * b) the page is already kmapped
	 */

	if (sblock->pagev[0].logical != le64_to_cpu(h->bytenr))
		++fail;

	if (sblock->pagev[0].generation != le64_to_cpu(h->generation))
		++fail;

	if (memcmp(h->fsid, fs_info->fsid, BTRFS_UUID_SIZE))
		++fail;

	if (memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
		   BTRFS_UUID_SIZE))
		++fail;

	BUG_ON(sdev->nodesize != sdev->leafsize);
	len = sdev->nodesize - BTRFS_CSUM_SIZE;
	mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
	p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
	index = 0;
	for (;;) {
		u64 l = min_t(u64, len, mapped_size);

		crc = btrfs_csum_data(root, p, crc, l);
		kunmap_atomic(mapped_buffer);
		len -= l;
		if (len == 0)
			break;
		index++;
		BUG_ON(index >= sblock->page_count);
		BUG_ON(!sblock->pagev[index].page);
		page = sblock->pagev[index].page;
		mapped_buffer = kmap_atomic(page);
		mapped_size = PAGE_SIZE;
		p = mapped_buffer;
	}

	btrfs_csum_final(crc, calculated_csum);
	if (memcmp(calculated_csum, on_disk_csum, sdev->csum_size))
		++crc_fail;

	return fail || crc_fail;
}

static int scrub_checksum_super(struct scrub_block *sblock)
{
	struct btrfs_super_block *s;
	struct scrub_dev *sdev = sblock->sdev;
	struct btrfs_root *root = sdev->dev->dev_root;
	struct btrfs_fs_info *fs_info = root->fs_info;
	u8 calculated_csum[BTRFS_CSUM_SIZE];
	u8 on_disk_csum[BTRFS_CSUM_SIZE];
	struct page *page;
	void *mapped_buffer;
	u64 mapped_size;
	void *p;
	u32 crc = ~(u32)0;
	int fail_gen = 0;
	int fail_cor = 0;
	u64 len;
	int index;

	BUG_ON(sblock->page_count < 1);
	page = sblock->pagev[0].page;
	mapped_buffer = kmap_atomic(page);
	s = (struct btrfs_super_block *)mapped_buffer;
	memcpy(on_disk_csum, s->csum, sdev->csum_size);

	if (sblock->pagev[0].logical != le64_to_cpu(s->bytenr))
		++fail_cor;

	if (sblock->pagev[0].generation != le64_to_cpu(s->generation))
		++fail_gen;

	if (memcmp(s->fsid, fs_info->fsid, BTRFS_UUID_SIZE))
		++fail_cor;

	len = BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE;
	mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
	p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
	index = 0;
	for (;;) {
		u64 l = min_t(u64, len, mapped_size);

		crc = btrfs_csum_data(root, p, crc, l);
		kunmap_atomic(mapped_buffer);
		len -= l;
		if (len == 0)
			break;
		index++;
		BUG_ON(index >= sblock->page_count);
		BUG_ON(!sblock->pagev[index].page);
		page = sblock->pagev[index].page;
		mapped_buffer = kmap_atomic(page);
		mapped_size = PAGE_SIZE;
		p = mapped_buffer;
	}

	btrfs_csum_final(crc, calculated_csum);
	if (memcmp(calculated_csum, on_disk_csum, sdev->csum_size))
		++fail_cor;

	if (fail_cor + fail_gen) {
		/*
		 * if we find an error in a super block, we just report it.
		 * They will get written with the next transaction commit
		 * anyway
		 */
		spin_lock(&sdev->stat_lock);
		++sdev->stat.super_errors;
		spin_unlock(&sdev->stat_lock);
		if (fail_cor)
			btrfs_dev_stat_inc_and_print(sdev->dev,
				BTRFS_DEV_STAT_CORRUPTION_ERRS);
		else
			btrfs_dev_stat_inc_and_print(sdev->dev,
				BTRFS_DEV_STAT_GENERATION_ERRS);
	}

	return fail_cor + fail_gen;
}

static void scrub_block_get(struct scrub_block *sblock)
{
	atomic_inc(&sblock->ref_count);
}

static void scrub_block_put(struct scrub_block *sblock)
{
	if (atomic_dec_and_test(&sblock->ref_count)) {
		int i;

		for (i = 0; i < sblock->page_count; i++)
			if (sblock->pagev[i].page)
				__free_page(sblock->pagev[i].page);
		kfree(sblock);
	}
}

static void scrub_submit(struct scrub_dev *sdev)
{
	struct scrub_bio *sbio;

	if (sdev->curr == -1)
		return;

	sbio = sdev->bios[sdev->curr];
	sdev->curr = -1;
	atomic_inc(&sdev->in_flight);

	btrfsic_submit_bio(READ, sbio->bio);
}

static int scrub_add_page_to_bio(struct scrub_dev *sdev,
				 struct scrub_page *spage)
{
	struct scrub_block *sblock = spage->sblock;
	struct scrub_bio *sbio;
	int ret;

again:
	/*
	 * grab a fresh bio or wait for one to become available
	 */
	while (sdev->curr == -1) {
		spin_lock(&sdev->list_lock);
		sdev->curr = sdev->first_free;
		if (sdev->curr != -1) {
			sdev->first_free = sdev->bios[sdev->curr]->next_free;
			sdev->bios[sdev->curr]->next_free = -1;
			sdev->bios[sdev->curr]->page_count = 0;
			spin_unlock(&sdev->list_lock);
		} else {
			spin_unlock(&sdev->list_lock);
			wait_event(sdev->list_wait, sdev->first_free != -1);
		}
	}
	sbio = sdev->bios[sdev->curr];
	if (sbio->page_count == 0) {
		struct bio *bio;

		sbio->physical = spage->physical;
		sbio->logical = spage->logical;
		bio = sbio->bio;
		if (!bio) {
			bio = bio_alloc(GFP_NOFS, sdev->pages_per_bio);
			if (!bio)
				return -ENOMEM;
			sbio->bio = bio;
		}

		bio->bi_private = sbio;
		bio->bi_end_io = scrub_bio_end_io;
		bio->bi_bdev = sdev->dev->bdev;
		bio->bi_sector = spage->physical >> 9;
		sbio->err = 0;
	} else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
		   spage->physical ||
		   sbio->logical + sbio->page_count * PAGE_SIZE !=
		   spage->logical) {
		scrub_submit(sdev);
		goto again;
	}

	sbio->pagev[sbio->page_count] = spage;
	ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
	if (ret != PAGE_SIZE) {
		if (sbio->page_count < 1) {
			bio_put(sbio->bio);
			sbio->bio = NULL;
			return -EIO;
		}
		scrub_submit(sdev);
		goto again;
	}

	scrub_block_get(sblock); /* one for the added page */
	atomic_inc(&sblock->outstanding_pages);
	sbio->page_count++;
	if (sbio->page_count == sdev->pages_per_bio)
		scrub_submit(sdev);

	return 0;
}

static int scrub_pages(struct scrub_dev *sdev, u64 logical, u64 len,
		       u64 physical, u64 flags, u64 gen, int mirror_num,
		       u8 *csum, int force)
{
	struct scrub_block *sblock;
	int index;

	sblock = kzalloc(sizeof(*sblock), GFP_NOFS);
	if (!sblock) {
		spin_lock(&sdev->stat_lock);
		sdev->stat.malloc_errors++;
		spin_unlock(&sdev->stat_lock);
		return -ENOMEM;
	}

	/* one ref inside this function, plus one for each page later on */
	atomic_set(&sblock->ref_count, 1);
	sblock->sdev = sdev;
	sblock->no_io_error_seen = 1;

	for (index = 0; len > 0; index++) {
		struct scrub_page *spage = sblock->pagev + index;
		u64 l = min_t(u64, len, PAGE_SIZE);

		BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
		spage->page = alloc_page(GFP_NOFS);
		if (!spage->page) {
			spin_lock(&sdev->stat_lock);
			sdev->stat.malloc_errors++;
			spin_unlock(&sdev->stat_lock);
			while (index > 0) {
				index--;
				__free_page(sblock->pagev[index].page);
			}
			kfree(sblock);
			return -ENOMEM;
		}
		spage->sblock = sblock;
		spage->dev = sdev->dev;
		spage->flags = flags;
		spage->generation = gen;
		spage->logical = logical;
		spage->physical = physical;
		spage->mirror_num = mirror_num;
		if (csum) {
			spage->have_csum = 1;
			memcpy(spage->csum, csum, sdev->csum_size);
		} else {
			spage->have_csum = 0;
		}
		sblock->page_count++;
		len -= l;
		logical += l;
		physical += l;
	}

	BUG_ON(sblock->page_count == 0);
	for (index = 0; index < sblock->page_count; index++) {
		struct scrub_page *spage = sblock->pagev + index;
		int ret;

		ret = scrub_add_page_to_bio(sdev, spage);
		if (ret) {
			scrub_block_put(sblock);
			return ret;
		}
	}

	if (force)
		scrub_submit(sdev);

	/* last one frees, either here or in bio completion for last page */
	scrub_block_put(sblock);
	return 0;
}

static void scrub_bio_end_io(struct bio *bio, int err)
{
	struct scrub_bio *sbio = bio->bi_private;
	struct scrub_dev *sdev = sbio->sdev;
	struct btrfs_fs_info *fs_info = sdev->dev->dev_root->fs_info;

	sbio->err = err;
	sbio->bio = bio;

	btrfs_queue_worker(&fs_info->scrub_workers, &sbio->work);
}

static void scrub_bio_end_io_worker(struct btrfs_work *work)
{
	struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
	struct scrub_dev *sdev = sbio->sdev;
	int i;

	BUG_ON(sbio->page_count > SCRUB_PAGES_PER_BIO);
	if (sbio->err) {
		for (i = 0; i < sbio->page_count; i++) {
			struct scrub_page *spage = sbio->pagev[i];

			spage->io_error = 1;
			spage->sblock->no_io_error_seen = 0;
		}
	}

	/* now complete the scrub_block items that have all pages completed */
	for (i = 0; i < sbio->page_count; i++) {
		struct scrub_page *spage = sbio->pagev[i];
		struct scrub_block *sblock = spage->sblock;

		if (atomic_dec_and_test(&sblock->outstanding_pages))
			scrub_block_complete(sblock);
		scrub_block_put(sblock);
	}

	if (sbio->err) {
		/* what is this good for??? */
		sbio->bio->bi_flags &= ~(BIO_POOL_MASK - 1);
		sbio->bio->bi_flags |= 1 << BIO_UPTODATE;
		sbio->bio->bi_phys_segments = 0;
		sbio->bio->bi_idx = 0;

		for (i = 0; i < sbio->page_count; i++) {
			struct bio_vec *bi;
			bi = &sbio->bio->bi_io_vec[i];
			bi->bv_offset = 0;
			bi->bv_len = PAGE_SIZE;
		}
	}

	bio_put(sbio->bio);
	sbio->bio = NULL;
	spin_lock(&sdev->list_lock);
	sbio->next_free = sdev->first_free;
	sdev->first_free = sbio->index;
	spin_unlock(&sdev->list_lock);
	atomic_dec(&sdev->in_flight);
	wake_up(&sdev->list_wait);
}

static void scrub_block_complete(struct scrub_block *sblock)
{
	if (!sblock->no_io_error_seen)
		scrub_handle_errored_block(sblock);
	else
		scrub_checksum(sblock);
}

static int scrub_find_csum(struct scrub_dev *sdev, u64 logical, u64 len,
			   u8 *csum)
{
	struct btrfs_ordered_sum *sum = NULL;
	int ret = 0;
	unsigned long i;
	unsigned long num_sectors;

	while (!list_empty(&sdev->csum_list)) {
		sum = list_first_entry(&sdev->csum_list,
				       struct btrfs_ordered_sum, list);
		if (sum->bytenr > logical)
			return 0;
		if (sum->bytenr + sum->len > logical)
			break;

		++sdev->stat.csum_discards;
		list_del(&sum->list);
		kfree(sum);
		sum = NULL;
	}
	if (!sum)
		return 0;

	num_sectors = sum->len / sdev->sectorsize;
	for (i = 0; i < num_sectors; ++i) {
		if (sum->sums[i].bytenr == logical) {
			memcpy(csum, &sum->sums[i].sum, sdev->csum_size);
			ret = 1;
			break;
		}
	}
	if (ret && i == num_sectors - 1) {
		list_del(&sum->list);
		kfree(sum);
	}
	return ret;
}

/* scrub extent tries to collect up to 64 kB for each bio */
static int scrub_extent(struct scrub_dev *sdev, u64 logical, u64 len,
			u64 physical, u64 flags, u64 gen, int mirror_num)
{
	int ret;
	u8 csum[BTRFS_CSUM_SIZE];
	u32 blocksize;

	if (flags & BTRFS_EXTENT_FLAG_DATA) {
		blocksize = sdev->sectorsize;
		spin_lock(&sdev->stat_lock);
		sdev->stat.data_extents_scrubbed++;
		sdev->stat.data_bytes_scrubbed += len;
		spin_unlock(&sdev->stat_lock);
	} else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
		BUG_ON(sdev->nodesize != sdev->leafsize);
		blocksize = sdev->nodesize;
		spin_lock(&sdev->stat_lock);
		sdev->stat.tree_extents_scrubbed++;
		sdev->stat.tree_bytes_scrubbed += len;
		spin_unlock(&sdev->stat_lock);
	} else {
		blocksize = sdev->sectorsize;
		BUG_ON(1);
	}

	while (len) {
		u64 l = min_t(u64, len, blocksize);
		int have_csum = 0;

		if (flags & BTRFS_EXTENT_FLAG_DATA) {
			/* push csums to sbio */
			have_csum = scrub_find_csum(sdev, logical, l, csum);
			if (have_csum == 0)
				++sdev->stat.no_csum;
		}
		ret = scrub_pages(sdev, logical, l, physical, flags, gen,
				  mirror_num, have_csum ? csum : NULL, 0);
		if (ret)
			return ret;
		len -= l;
		logical += l;
		physical += l;
	}
	return 0;
}

static noinline_for_stack int scrub_stripe(struct scrub_dev *sdev,
	struct map_lookup *map, int num, u64 base, u64 length)
{
	struct btrfs_path *path;
	struct btrfs_fs_info *fs_info = sdev->dev->dev_root->fs_info;
	struct btrfs_root *root = fs_info->extent_root;
	struct btrfs_root *csum_root = fs_info->csum_root;
	struct btrfs_extent_item *extent;
	struct blk_plug plug;
	u64 flags;
	int ret;
	int slot;
	int i;
	u64 nstripes;
	struct extent_buffer *l;
	struct btrfs_key key;
	u64 physical;
	u64 logical;
	u64 generation;
	int mirror_num;
	struct reada_control *reada1;
	struct reada_control *reada2;
	struct btrfs_key key_start;
	struct btrfs_key key_end;

	u64 increment = map->stripe_len;
	u64 offset;

	nstripes = length;
	offset = 0;
	do_div(nstripes, map->stripe_len);
	if (map->type & BTRFS_BLOCK_GROUP_RAID0) {
		offset = map->stripe_len * num;
		increment = map->stripe_len * map->num_stripes;
		mirror_num = 1;
	} else if (map->type & BTRFS_BLOCK_GROUP_RAID10) {
		int factor = map->num_stripes / map->sub_stripes;
		offset = map->stripe_len * (num / map->sub_stripes);
		increment = map->stripe_len * factor;
		mirror_num = num % map->sub_stripes + 1;
	} else if (map->type & BTRFS_BLOCK_GROUP_RAID1) {
		increment = map->stripe_len;
		mirror_num = num % map->num_stripes + 1;
	} else if (map->type & BTRFS_BLOCK_GROUP_DUP) {
		increment = map->stripe_len;
		mirror_num = num % map->num_stripes + 1;
	} else {
		increment = map->stripe_len;
		mirror_num = 1;
	}

	path = btrfs_alloc_path();
	if (!path)
		return -ENOMEM;

	/*
	 * work on commit root. The related disk blocks are static as
	 * long as COW is applied. This means, it is save to rewrite
	 * them to repair disk errors without any race conditions
	 */
	path->search_commit_root = 1;
	path->skip_locking = 1;

	/*
	 * trigger the readahead for extent tree csum tree and wait for
	 * completion. During readahead, the scrub is officially paused
	 * to not hold off transaction commits
	 */
	logical = base + offset;

	wait_event(sdev->list_wait,
		   atomic_read(&sdev->in_flight) == 0);
	atomic_inc(&fs_info->scrubs_paused);
	wake_up(&fs_info->scrub_pause_wait);

	/* FIXME it might be better to start readahead at commit root */
	key_start.objectid = logical;
	key_start.type = BTRFS_EXTENT_ITEM_KEY;
	key_start.offset = (u64)0;
	key_end.objectid = base + offset + nstripes * increment;
	key_end.type = BTRFS_EXTENT_ITEM_KEY;
	key_end.offset = (u64)0;
	reada1 = btrfs_reada_add(root, &key_start, &key_end);

	key_start.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
	key_start.type = BTRFS_EXTENT_CSUM_KEY;
	key_start.offset = logical;
	key_end.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
	key_end.type = BTRFS_EXTENT_CSUM_KEY;
	key_end.offset = base + offset + nstripes * increment;
	reada2 = btrfs_reada_add(csum_root, &key_start, &key_end);

	if (!IS_ERR(reada1))
		btrfs_reada_wait(reada1);
	if (!IS_ERR(reada2))
		btrfs_reada_wait(reada2);

	mutex_lock(&fs_info->scrub_lock);
	while (atomic_read(&fs_info->scrub_pause_req)) {
		mutex_unlock(&fs_info->scrub_lock);
		wait_event(fs_info->scrub_pause_wait,
		   atomic_read(&fs_info->scrub_pause_req) == 0);
		mutex_lock(&fs_info->scrub_lock);
	}
	atomic_dec(&fs_info->scrubs_paused);
	mutex_unlock(&fs_info->scrub_lock);
	wake_up(&fs_info->scrub_pause_wait);

	/*
	 * collect all data csums for the stripe to avoid seeking during
	 * the scrub. This might currently (crc32) end up to be about 1MB
	 */
	blk_start_plug(&plug);

	/*
	 * now find all extents for each stripe and scrub them
	 */
	logical = base + offset;
	physical = map->stripes[num].physical;
	ret = 0;
	for (i = 0; i < nstripes; ++i) {
		/*
		 * canceled?
		 */
		if (atomic_read(&fs_info->scrub_cancel_req) ||
		    atomic_read(&sdev->cancel_req)) {
			ret = -ECANCELED;
			goto out;
		}
		/*
		 * check to see if we have to pause
		 */
		if (atomic_read(&fs_info->scrub_pause_req)) {
			/* push queued extents */
			scrub_submit(sdev);
			wait_event(sdev->list_wait,
				   atomic_read(&sdev->in_flight) == 0);
			atomic_inc(&fs_info->scrubs_paused);
			wake_up(&fs_info->scrub_pause_wait);
			mutex_lock(&fs_info->scrub_lock);
			while (atomic_read(&fs_info->scrub_pause_req)) {
				mutex_unlock(&fs_info->scrub_lock);
				wait_event(fs_info->scrub_pause_wait,
				   atomic_read(&fs_info->scrub_pause_req) == 0);
				mutex_lock(&fs_info->scrub_lock);
			}
			atomic_dec(&fs_info->scrubs_paused);
			mutex_unlock(&fs_info->scrub_lock);
			wake_up(&fs_info->scrub_pause_wait);
		}

		ret = btrfs_lookup_csums_range(csum_root, logical,
					       logical + map->stripe_len - 1,
					       &sdev->csum_list, 1);
		if (ret)
			goto out;

		key.objectid = logical;
		key.type = BTRFS_EXTENT_ITEM_KEY;
		key.offset = (u64)0;

		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
		if (ret < 0)
			goto out;
		if (ret > 0) {
			ret = btrfs_previous_item(root, path, 0,
						  BTRFS_EXTENT_ITEM_KEY);
			if (ret < 0)
				goto out;
			if (ret > 0) {
				/* there's no smaller item, so stick with the
				 * larger one */
				btrfs_release_path(path);
				ret = btrfs_search_slot(NULL, root, &key,
							path, 0, 0);
				if (ret < 0)
					goto out;
			}
		}

		while (1) {
			l = path->nodes[0];
			slot = path->slots[0];
			if (slot >= btrfs_header_nritems(l)) {
				ret = btrfs_next_leaf(root, path);
				if (ret == 0)
					continue;
				if (ret < 0)
					goto out;

				break;
			}
			btrfs_item_key_to_cpu(l, &key, slot);

			if (key.objectid + key.offset <= logical)
				goto next;

			if (key.objectid >= logical + map->stripe_len)
				break;

			if (btrfs_key_type(&key) != BTRFS_EXTENT_ITEM_KEY)
				goto next;

			extent = btrfs_item_ptr(l, slot,
						struct btrfs_extent_item);
			flags = btrfs_extent_flags(l, extent);
			generation = btrfs_extent_generation(l, extent);

			if (key.objectid < logical &&
			    (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)) {
				printk(KERN_ERR
				       "btrfs scrub: tree block %llu spanning "
				       "stripes, ignored. logical=%llu\n",
				       (unsigned long long)key.objectid,
				       (unsigned long long)logical);
				goto next;
			}

			/*
			 * trim extent to this stripe
			 */
			if (key.objectid < logical) {
				key.offset -= logical - key.objectid;
				key.objectid = logical;
			}
			if (key.objectid + key.offset >
			    logical + map->stripe_len) {
				key.offset = logical + map->stripe_len -
					     key.objectid;
			}

			ret = scrub_extent(sdev, key.objectid, key.offset,
					   key.objectid - logical + physical,
					   flags, generation, mirror_num);
			if (ret)
				goto out;

next:
			path->slots[0]++;
		}
		btrfs_release_path(path);
		logical += increment;
		physical += map->stripe_len;
		spin_lock(&sdev->stat_lock);
		sdev->stat.last_physical = physical;
		spin_unlock(&sdev->stat_lock);
	}
	/* push queued extents */
	scrub_submit(sdev);

out:
	blk_finish_plug(&plug);
	btrfs_free_path(path);
	return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_chunk(struct scrub_dev *sdev,
	u64 chunk_tree, u64 chunk_objectid, u64 chunk_offset, u64 length,
	u64 dev_offset)
{
	struct btrfs_mapping_tree *map_tree =
		&sdev->dev->dev_root->fs_info->mapping_tree;
	struct map_lookup *map;
	struct extent_map *em;
	int i;
	int ret = -EINVAL;

	read_lock(&map_tree->map_tree.lock);
	em = lookup_extent_mapping(&map_tree->map_tree, chunk_offset, 1);
	read_unlock(&map_tree->map_tree.lock);

	if (!em)
		return -EINVAL;

	map = (struct map_lookup *)em->bdev;
	if (em->start != chunk_offset)
		goto out;

	if (em->len < length)
		goto out;

	for (i = 0; i < map->num_stripes; ++i) {
		if (map->stripes[i].dev == sdev->dev &&
		    map->stripes[i].physical == dev_offset) {
			ret = scrub_stripe(sdev, map, i, chunk_offset, length);
			if (ret)
				goto out;
		}
	}
out:
	free_extent_map(em);

	return ret;
}

static noinline_for_stack
int scrub_enumerate_chunks(struct scrub_dev *sdev, u64 start, u64 end)
{
	struct btrfs_dev_extent *dev_extent = NULL;
	struct btrfs_path *path;
	struct btrfs_root *root = sdev->dev->dev_root;
	struct btrfs_fs_info *fs_info = root->fs_info;
	u64 length;
	u64 chunk_tree;
	u64 chunk_objectid;
	u64 chunk_offset;
	int ret;
	int slot;
	struct extent_buffer *l;
	struct btrfs_key key;
	struct btrfs_key found_key;
	struct btrfs_block_group_cache *cache;

	path = btrfs_alloc_path();
	if (!path)
		return -ENOMEM;

	path->reada = 2;
	path->search_commit_root = 1;
	path->skip_locking = 1;

	key.objectid = sdev->dev->devid;
	key.offset = 0ull;
	key.type = BTRFS_DEV_EXTENT_KEY;


	while (1) {
		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
		if (ret < 0)
			break;
		if (ret > 0) {
			if (path->slots[0] >=
			    btrfs_header_nritems(path->nodes[0])) {
				ret = btrfs_next_leaf(root, path);
				if (ret)
					break;
			}
		}

		l = path->nodes[0];
		slot = path->slots[0];

		btrfs_item_key_to_cpu(l, &found_key, slot);

		if (found_key.objectid != sdev->dev->devid)
			break;

		if (btrfs_key_type(&found_key) != BTRFS_DEV_EXTENT_KEY)
			break;

		if (found_key.offset >= end)
			break;

		if (found_key.offset < key.offset)
			break;

		dev_extent = btrfs_item_ptr(l, slot, struct btrfs_dev_extent);
		length = btrfs_dev_extent_length(l, dev_extent);

		if (found_key.offset + length <= start) {
			key.offset = found_key.offset + length;
			btrfs_release_path(path);
			continue;
		}

		chunk_tree = btrfs_dev_extent_chunk_tree(l, dev_extent);
		chunk_objectid = btrfs_dev_extent_chunk_objectid(l, dev_extent);
		chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);

		/*
		 * get a reference on the corresponding block group to prevent
		 * the chunk from going away while we scrub it
		 */
		cache = btrfs_lookup_block_group(fs_info, chunk_offset);
		if (!cache) {
			ret = -ENOENT;
			break;
		}
		ret = scrub_chunk(sdev, chunk_tree, chunk_objectid,
				  chunk_offset, length, found_key.offset);
		btrfs_put_block_group(cache);
		if (ret)
			break;

		key.offset = found_key.offset + length;
		btrfs_release_path(path);
	}

	btrfs_free_path(path);

	/*
	 * ret can still be 1 from search_slot or next_leaf,
	 * that's not an error
	 */
	return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_supers(struct scrub_dev *sdev)
{
	int	i;
	u64	bytenr;
	u64	gen;
	int	ret;
	struct btrfs_device *device = sdev->dev;
	struct btrfs_root *root = device->dev_root;

	if (root->fs_info->fs_state & BTRFS_SUPER_FLAG_ERROR)
		return -EIO;

	gen = root->fs_info->last_trans_committed;

	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
		bytenr = btrfs_sb_offset(i);
		if (bytenr + BTRFS_SUPER_INFO_SIZE > device->total_bytes)
			break;

		ret = scrub_pages(sdev, bytenr, BTRFS_SUPER_INFO_SIZE, bytenr,
				     BTRFS_EXTENT_FLAG_SUPER, gen, i, NULL, 1);
		if (ret)
			return ret;
	}
	wait_event(sdev->list_wait, atomic_read(&sdev->in_flight) == 0);

	return 0;
}

/*
 * get a reference count on fs_info->scrub_workers. start worker if necessary
 */
static noinline_for_stack int scrub_workers_get(struct btrfs_root *root)
{
	struct btrfs_fs_info *fs_info = root->fs_info;
	int ret = 0;

	mutex_lock(&fs_info->scrub_lock);
	if (fs_info->scrub_workers_refcnt == 0) {
		btrfs_init_workers(&fs_info->scrub_workers, "scrub",
			   fs_info->thread_pool_size, &fs_info->generic_worker);
		fs_info->scrub_workers.idle_thresh = 4;
		ret = btrfs_start_workers(&fs_info->scrub_workers);
		if (ret)
			goto out;
	}
	++fs_info->scrub_workers_refcnt;
out:
	mutex_unlock(&fs_info->scrub_lock);

	return ret;
}

static noinline_for_stack void scrub_workers_put(struct btrfs_root *root)
{
	struct btrfs_fs_info *fs_info = root->fs_info;

	mutex_lock(&fs_info->scrub_lock);
	if (--fs_info->scrub_workers_refcnt == 0)
		btrfs_stop_workers(&fs_info->scrub_workers);
	WARN_ON(fs_info->scrub_workers_refcnt < 0);
	mutex_unlock(&fs_info->scrub_lock);
}


int btrfs_scrub_dev(struct btrfs_root *root, u64 devid, u64 start, u64 end,
		    struct btrfs_scrub_progress *progress, int readonly)
{
	struct scrub_dev *sdev;
	struct btrfs_fs_info *fs_info = root->fs_info;
	int ret;
	struct btrfs_device *dev;

	if (btrfs_fs_closing(root->fs_info))
		return -EINVAL;

	/*
	 * check some assumptions
	 */
	if (root->nodesize != root->leafsize) {
		printk(KERN_ERR
		       "btrfs_scrub: size assumption nodesize == leafsize (%d == %d) fails\n",
		       root->nodesize, root->leafsize);
		return -EINVAL;
	}

	if (root->nodesize > BTRFS_STRIPE_LEN) {
		/*
		 * in this case scrub is unable to calculate the checksum
		 * the way scrub is implemented. Do not handle this
		 * situation at all because it won't ever happen.
		 */
		printk(KERN_ERR
		       "btrfs_scrub: size assumption nodesize <= BTRFS_STRIPE_LEN (%d <= %d) fails\n",
		       root->nodesize, BTRFS_STRIPE_LEN);
		return -EINVAL;
	}

	if (root->sectorsize != PAGE_SIZE) {
		/* not supported for data w/o checksums */
		printk(KERN_ERR
		       "btrfs_scrub: size assumption sectorsize != PAGE_SIZE (%d != %lld) fails\n",
		       root->sectorsize, (unsigned long long)PAGE_SIZE);
		return -EINVAL;
	}

	ret = scrub_workers_get(root);
	if (ret)
		return ret;

	mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
	dev = btrfs_find_device(root, devid, NULL, NULL);
	if (!dev || dev->missing) {
		mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
		scrub_workers_put(root);
		return -ENODEV;
	}
	mutex_lock(&fs_info->scrub_lock);

	if (!dev->in_fs_metadata) {
		mutex_unlock(&fs_info->scrub_lock);
		mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
		scrub_workers_put(root);
		return -ENODEV;
	}

	if (dev->scrub_device) {
		mutex_unlock(&fs_info->scrub_lock);
		mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
		scrub_workers_put(root);
		return -EINPROGRESS;
	}
	sdev = scrub_setup_dev(dev);
	if (IS_ERR(sdev)) {
		mutex_unlock(&fs_info->scrub_lock);
		mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
		scrub_workers_put(root);
		return PTR_ERR(sdev);
	}
	sdev->readonly = readonly;
	dev->scrub_device = sdev;

	atomic_inc(&fs_info->scrubs_running);
	mutex_unlock(&fs_info->scrub_lock);
	mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);

	down_read(&fs_info->scrub_super_lock);
	ret = scrub_supers(sdev);
	up_read(&fs_info->scrub_super_lock);

	if (!ret)
		ret = scrub_enumerate_chunks(sdev, start, end);

	wait_event(sdev->list_wait, atomic_read(&sdev->in_flight) == 0);
	atomic_dec(&fs_info->scrubs_running);
	wake_up(&fs_info->scrub_pause_wait);

	wait_event(sdev->list_wait, atomic_read(&sdev->fixup_cnt) == 0);

	if (progress)
		memcpy(progress, &sdev->stat, sizeof(*progress));

	mutex_lock(&fs_info->scrub_lock);
	dev->scrub_device = NULL;
	mutex_unlock(&fs_info->scrub_lock);

	scrub_free_dev(sdev);
	scrub_workers_put(root);

	return ret;
}

void btrfs_scrub_pause(struct btrfs_root *root)
{
	struct btrfs_fs_info *fs_info = root->fs_info;

	mutex_lock(&fs_info->scrub_lock);
	atomic_inc(&fs_info->scrub_pause_req);
	while (atomic_read(&fs_info->scrubs_paused) !=
	       atomic_read(&fs_info->scrubs_running)) {
		mutex_unlock(&fs_info->scrub_lock);
		wait_event(fs_info->scrub_pause_wait,
			   atomic_read(&fs_info->scrubs_paused) ==
			   atomic_read(&fs_info->scrubs_running));
		mutex_lock(&fs_info->scrub_lock);
	}
	mutex_unlock(&fs_info->scrub_lock);
}

void btrfs_scrub_continue(struct btrfs_root *root)
{
	struct btrfs_fs_info *fs_info = root->fs_info;

	atomic_dec(&fs_info->scrub_pause_req);
	wake_up(&fs_info->scrub_pause_wait);
}

void btrfs_scrub_pause_super(struct btrfs_root *root)
{
	down_write(&root->fs_info->scrub_super_lock);
}

void btrfs_scrub_continue_super(struct btrfs_root *root)
{
	up_write(&root->fs_info->scrub_super_lock);
}

int __btrfs_scrub_cancel(struct btrfs_fs_info *fs_info)
{

	mutex_lock(&fs_info->scrub_lock);
	if (!atomic_read(&fs_info->scrubs_running)) {
		mutex_unlock(&fs_info->scrub_lock);
		return -ENOTCONN;
	}

	atomic_inc(&fs_info->scrub_cancel_req);
	while (atomic_read(&fs_info->scrubs_running)) {
		mutex_unlock(&fs_info->scrub_lock);
		wait_event(fs_info->scrub_pause_wait,
			   atomic_read(&fs_info->scrubs_running) == 0);
		mutex_lock(&fs_info->scrub_lock);
	}
	atomic_dec(&fs_info->scrub_cancel_req);
	mutex_unlock(&fs_info->scrub_lock);

	return 0;
}

int btrfs_scrub_cancel(struct btrfs_root *root)
{
	return __btrfs_scrub_cancel(root->fs_info);
}

int btrfs_scrub_cancel_dev(struct btrfs_root *root, struct btrfs_device *dev)
{
	struct btrfs_fs_info *fs_info = root->fs_info;
	struct scrub_dev *sdev;

	mutex_lock(&fs_info->scrub_lock);
	sdev = dev->scrub_device;
	if (!sdev) {
		mutex_unlock(&fs_info->scrub_lock);
		return -ENOTCONN;
	}
	atomic_inc(&sdev->cancel_req);
	while (dev->scrub_device) {
		mutex_unlock(&fs_info->scrub_lock);
		wait_event(fs_info->scrub_pause_wait,
			   dev->scrub_device == NULL);
		mutex_lock(&fs_info->scrub_lock);
	}
	mutex_unlock(&fs_info->scrub_lock);

	return 0;
}

int btrfs_scrub_cancel_devid(struct btrfs_root *root, u64 devid)
{
	struct btrfs_fs_info *fs_info = root->fs_info;
	struct btrfs_device *dev;
	int ret;

	/*
	 * we have to hold the device_list_mutex here so the device
	 * does not go away in cancel_dev. FIXME: find a better solution
	 */
	mutex_lock(&fs_info->fs_devices->device_list_mutex);
	dev = btrfs_find_device(root, devid, NULL, NULL);
	if (!dev) {
		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
		return -ENODEV;
	}
	ret = btrfs_scrub_cancel_dev(root, dev);
	mutex_unlock(&fs_info->fs_devices->device_list_mutex);

	return ret;
}

int btrfs_scrub_progress(struct btrfs_root *root, u64 devid,
			 struct btrfs_scrub_progress *progress)
{
	struct btrfs_device *dev;
	struct scrub_dev *sdev = NULL;

	mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
	dev = btrfs_find_device(root, devid, NULL, NULL);
	if (dev)
		sdev = dev->scrub_device;
	if (sdev)
		memcpy(progress, &sdev->stat, sizeof(*progress));
	mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);

	return dev ? (sdev ? 0 : -ENOTCONN) : -ENODEV;
}