OpenCloudOS-Kernel/drivers/mtd/mtdswap.c

/*
 * Swap block device support for MTDs
 * Turns an MTD device into a swap device with block wear leveling
 *
 * Copyright © 2007,2011 Nokia Corporation. All rights reserved.
 *
 * Authors: Jarkko Lavinen <jarkko.lavinen@nokia.com>
 *
 * Based on Richard Purdie's earlier implementation in 2007. Background
 * support and lock-less operation written by Adrian Hunter.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 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., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/mtd/mtd.h>
#include <linux/mtd/blktrans.h>
#include <linux/rbtree.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/genhd.h>
#include <linux/swap.h>
#include <linux/debugfs.h>
#include <linux/seq_file.h>
#include <linux/device.h>
#include <linux/math64.h>

#define MTDSWAP_PREFIX "mtdswap"

/*
 * The number of free eraseblocks when GC should stop
 */
#define CLEAN_BLOCK_THRESHOLD	20

/*
 * Number of free eraseblocks below which GC can also collect low frag
 * blocks.
 */
#define LOW_FRAG_GC_THRESHOLD	5

/*
 * Wear level cost amortization. We want to do wear leveling on the background
 * without disturbing gc too much. This is made by defining max GC frequency.
 * Frequency value 6 means 1/6 of the GC passes will pick an erase block based
 * on the biggest wear difference rather than the biggest dirtiness.
 *
 * The lower freq2 should be chosen so that it makes sure the maximum erase
 * difference will decrease even if a malicious application is deliberately
 * trying to make erase differences large.
 */
#define MAX_ERASE_DIFF		4000
#define COLLECT_NONDIRTY_BASE	MAX_ERASE_DIFF
#define COLLECT_NONDIRTY_FREQ1	6
#define COLLECT_NONDIRTY_FREQ2	4

#define PAGE_UNDEF		UINT_MAX
#define BLOCK_UNDEF		UINT_MAX
#define BLOCK_ERROR		(UINT_MAX - 1)
#define BLOCK_MAX		(UINT_MAX - 2)

#define EBLOCK_BAD		(1 << 0)
#define EBLOCK_NOMAGIC		(1 << 1)
#define EBLOCK_BITFLIP		(1 << 2)
#define EBLOCK_FAILED		(1 << 3)
#define EBLOCK_READERR		(1 << 4)
#define EBLOCK_IDX_SHIFT	5

struct swap_eb {
	struct rb_node rb;
	struct rb_root *root;

	unsigned int flags;
	unsigned int active_count;
	unsigned int erase_count;
	unsigned int pad;		/* speeds up pointer decrement */
};

#define MTDSWAP_ECNT_MIN(rbroot) (rb_entry(rb_first(rbroot), struct swap_eb, \
				rb)->erase_count)
#define MTDSWAP_ECNT_MAX(rbroot) (rb_entry(rb_last(rbroot), struct swap_eb, \
				rb)->erase_count)

struct mtdswap_tree {
	struct rb_root root;
	unsigned int count;
};

enum {
	MTDSWAP_CLEAN,
	MTDSWAP_USED,
	MTDSWAP_LOWFRAG,
	MTDSWAP_HIFRAG,
	MTDSWAP_DIRTY,
	MTDSWAP_BITFLIP,
	MTDSWAP_FAILING,
	MTDSWAP_TREE_CNT,
};

struct mtdswap_dev {
	struct mtd_blktrans_dev *mbd_dev;
	struct mtd_info *mtd;
	struct device *dev;

	unsigned int *page_data;
	unsigned int *revmap;

	unsigned int eblks;
	unsigned int spare_eblks;
	unsigned int pages_per_eblk;
	unsigned int max_erase_count;
	struct swap_eb *eb_data;

	struct mtdswap_tree trees[MTDSWAP_TREE_CNT];

	unsigned long long sect_read_count;
	unsigned long long sect_write_count;
	unsigned long long mtd_write_count;
	unsigned long long mtd_read_count;
	unsigned long long discard_count;
	unsigned long long discard_page_count;

	unsigned int curr_write_pos;
	struct swap_eb *curr_write;

	char *page_buf;
	char *oob_buf;
};

struct mtdswap_oobdata {
	__le16 magic;
	__le32 count;
} __packed;

#define MTDSWAP_MAGIC_CLEAN	0x2095
#define MTDSWAP_MAGIC_DIRTY	(MTDSWAP_MAGIC_CLEAN + 1)
#define MTDSWAP_TYPE_CLEAN	0
#define MTDSWAP_TYPE_DIRTY	1
#define MTDSWAP_OOBSIZE		sizeof(struct mtdswap_oobdata)

#define MTDSWAP_ERASE_RETRIES	3 /* Before marking erase block bad */
#define MTDSWAP_IO_RETRIES	3

enum {
	MTDSWAP_SCANNED_CLEAN,
	MTDSWAP_SCANNED_DIRTY,
	MTDSWAP_SCANNED_BITFLIP,
	MTDSWAP_SCANNED_BAD,
};

/*
 * In the worst case mtdswap_writesect() has allocated the last clean
 * page from the current block and is then pre-empted by the GC
 * thread. The thread can consume a full erase block when moving a
 * block.
 */
#define MIN_SPARE_EBLOCKS	2
#define MIN_ERASE_BLOCKS	(MIN_SPARE_EBLOCKS + 1)

#define TREE_ROOT(d, name) (&d->trees[MTDSWAP_ ## name].root)
#define TREE_EMPTY(d, name) (TREE_ROOT(d, name)->rb_node == NULL)
#define TREE_NONEMPTY(d, name) (!TREE_EMPTY(d, name))
#define TREE_COUNT(d, name) (d->trees[MTDSWAP_ ## name].count)

#define MTDSWAP_MBD_TO_MTDSWAP(dev) ((struct mtdswap_dev *)dev->priv)

static char partitions[128] = "";
module_param_string(partitions, partitions, sizeof(partitions), 0444);
MODULE_PARM_DESC(partitions, "MTD partition numbers to use as swap "
		"partitions=\"1,3,5\"");

static unsigned int spare_eblocks = 10;
module_param(spare_eblocks, uint, 0444);
MODULE_PARM_DESC(spare_eblocks, "Percentage of spare erase blocks for "
		"garbage collection (default 10%)");

static bool header; /* false */
module_param(header, bool, 0444);
MODULE_PARM_DESC(header,
		"Include builtin swap header (default 0, without header)");

static int mtdswap_gc(struct mtdswap_dev *d, unsigned int background);

static loff_t mtdswap_eb_offset(struct mtdswap_dev *d, struct swap_eb *eb)
{
	return (loff_t)(eb - d->eb_data) * d->mtd->erasesize;
}

static void mtdswap_eb_detach(struct mtdswap_dev *d, struct swap_eb *eb)
{
	unsigned int oldidx;
	struct mtdswap_tree *tp;

	if (eb->root) {
		tp = container_of(eb->root, struct mtdswap_tree, root);
		oldidx = tp - &d->trees[0];

		d->trees[oldidx].count--;
		rb_erase(&eb->rb, eb->root);
	}
}

static void __mtdswap_rb_add(struct rb_root *root, struct swap_eb *eb)
{
	struct rb_node **p, *parent = NULL;
	struct swap_eb *cur;

	p = &root->rb_node;
	while (*p) {
		parent = *p;
		cur = rb_entry(parent, struct swap_eb, rb);
		if (eb->erase_count > cur->erase_count)
			p = &(*p)->rb_right;
		else
			p = &(*p)->rb_left;
	}

	rb_link_node(&eb->rb, parent, p);
	rb_insert_color(&eb->rb, root);
}

static void mtdswap_rb_add(struct mtdswap_dev *d, struct swap_eb *eb, int idx)
{
	struct rb_root *root;

	if (eb->root == &d->trees[idx].root)
		return;

	mtdswap_eb_detach(d, eb);
	root = &d->trees[idx].root;
	__mtdswap_rb_add(root, eb);
	eb->root = root;
	d->trees[idx].count++;
}

static struct rb_node *mtdswap_rb_index(struct rb_root *root, unsigned int idx)
{
	struct rb_node *p;
	unsigned int i;

	p = rb_first(root);
	i = 0;
	while (i < idx && p) {
		p = rb_next(p);
		i++;
	}

	return p;
}

static int mtdswap_handle_badblock(struct mtdswap_dev *d, struct swap_eb *eb)
{
	int ret;
	loff_t offset;

	d->spare_eblks--;
	eb->flags |= EBLOCK_BAD;
	mtdswap_eb_detach(d, eb);
	eb->root = NULL;

	/* badblocks not supported */
	if (!mtd_can_have_bb(d->mtd))
		return 1;

	offset = mtdswap_eb_offset(d, eb);
	dev_warn(d->dev, "Marking bad block at %08llx\n", offset);
	ret = mtd_block_markbad(d->mtd, offset);

	if (ret) {
		dev_warn(d->dev, "Mark block bad failed for block at %08llx "
			"error %d\n", offset, ret);
		return ret;
	}

	return 1;

}

static int mtdswap_handle_write_error(struct mtdswap_dev *d, struct swap_eb *eb)
{
	unsigned int marked = eb->flags & EBLOCK_FAILED;
	struct swap_eb *curr_write = d->curr_write;

	eb->flags |= EBLOCK_FAILED;
	if (curr_write == eb) {
		d->curr_write = NULL;

		if (!marked && d->curr_write_pos != 0) {
			mtdswap_rb_add(d, eb, MTDSWAP_FAILING);
			return 0;
		}
	}

	return mtdswap_handle_badblock(d, eb);
}

static int mtdswap_read_oob(struct mtdswap_dev *d, loff_t from,
			struct mtd_oob_ops *ops)
{
	int ret = mtd_read_oob(d->mtd, from, ops);

	if (mtd_is_bitflip(ret))
		return ret;

	if (ret) {
		dev_warn(d->dev, "Read OOB failed %d for block at %08llx\n",
			ret, from);
		return ret;
	}

	if (ops->oobretlen < ops->ooblen) {
		dev_warn(d->dev, "Read OOB return short read (%zd bytes not "
			"%zd) for block at %08llx\n",
			ops->oobretlen, ops->ooblen, from);
		return -EIO;
	}

	return 0;
}

static int mtdswap_read_markers(struct mtdswap_dev *d, struct swap_eb *eb)
{
	struct mtdswap_oobdata *data, *data2;
	int ret;
	loff_t offset;
	struct mtd_oob_ops ops;

	offset = mtdswap_eb_offset(d, eb);

	/* Check first if the block is bad. */
	if (mtd_can_have_bb(d->mtd) && mtd_block_isbad(d->mtd, offset))
		return MTDSWAP_SCANNED_BAD;

	ops.ooblen = 2 * d->mtd->oobavail;
	ops.oobbuf = d->oob_buf;
	ops.ooboffs = 0;
	ops.datbuf = NULL;
	ops.mode = MTD_OPS_AUTO_OOB;

	ret = mtdswap_read_oob(d, offset, &ops);

	if (ret && !mtd_is_bitflip(ret))
		return ret;

	data = (struct mtdswap_oobdata *)d->oob_buf;
	data2 = (struct mtdswap_oobdata *)
		(d->oob_buf + d->mtd->oobavail);

	if (le16_to_cpu(data->magic) == MTDSWAP_MAGIC_CLEAN) {
		eb->erase_count = le32_to_cpu(data->count);
		if (mtd_is_bitflip(ret))
			ret = MTDSWAP_SCANNED_BITFLIP;
		else {
			if (le16_to_cpu(data2->magic) == MTDSWAP_MAGIC_DIRTY)
				ret = MTDSWAP_SCANNED_DIRTY;
			else
				ret = MTDSWAP_SCANNED_CLEAN;
		}
	} else {
		eb->flags |= EBLOCK_NOMAGIC;
		ret = MTDSWAP_SCANNED_DIRTY;
	}

	return ret;
}

static int mtdswap_write_marker(struct mtdswap_dev *d, struct swap_eb *eb,
				u16 marker)
{
	struct mtdswap_oobdata n;
	int ret;
	loff_t offset;
	struct mtd_oob_ops ops;

	ops.ooboffs = 0;
	ops.oobbuf = (uint8_t *)&n;
	ops.mode = MTD_OPS_AUTO_OOB;
	ops.datbuf = NULL;

	if (marker == MTDSWAP_TYPE_CLEAN) {
		n.magic = cpu_to_le16(MTDSWAP_MAGIC_CLEAN);
		n.count = cpu_to_le32(eb->erase_count);
		ops.ooblen = MTDSWAP_OOBSIZE;
		offset = mtdswap_eb_offset(d, eb);
	} else {
		n.magic = cpu_to_le16(MTDSWAP_MAGIC_DIRTY);
		ops.ooblen = sizeof(n.magic);
		offset = mtdswap_eb_offset(d, eb) + d->mtd->writesize;
	}

	ret = mtd_write_oob(d->mtd, offset, &ops);

	if (ret) {
		dev_warn(d->dev, "Write OOB failed for block at %08llx "
			"error %d\n", offset, ret);
		if (ret == -EIO || mtd_is_eccerr(ret))
			mtdswap_handle_write_error(d, eb);
		return ret;
	}

	if (ops.oobretlen != ops.ooblen) {
		dev_warn(d->dev, "Short OOB write for block at %08llx: "
			"%zd not %zd\n",
			offset, ops.oobretlen, ops.ooblen);
		return ret;
	}

	return 0;
}

/*
 * Are there any erase blocks without MAGIC_CLEAN header, presumably
 * because power was cut off after erase but before header write? We
 * need to guestimate the erase count.
 */
static void mtdswap_check_counts(struct mtdswap_dev *d)
{
	struct rb_root hist_root = RB_ROOT;
	struct rb_node *medrb;
	struct swap_eb *eb;
	unsigned int i, cnt, median;

	cnt = 0;
	for (i = 0; i < d->eblks; i++) {
		eb = d->eb_data + i;

		if (eb->flags & (EBLOCK_NOMAGIC | EBLOCK_BAD | EBLOCK_READERR))
			continue;

		__mtdswap_rb_add(&hist_root, eb);
		cnt++;
	}

	if (cnt == 0)
		return;

	medrb = mtdswap_rb_index(&hist_root, cnt / 2);
	median = rb_entry(medrb, struct swap_eb, rb)->erase_count;

	d->max_erase_count = MTDSWAP_ECNT_MAX(&hist_root);

	for (i = 0; i < d->eblks; i++) {
		eb = d->eb_data + i;

		if (eb->flags & (EBLOCK_NOMAGIC | EBLOCK_READERR))
			eb->erase_count = median;

		if (eb->flags & (EBLOCK_NOMAGIC | EBLOCK_BAD | EBLOCK_READERR))
			continue;

		rb_erase(&eb->rb, &hist_root);
	}
}

static void mtdswap_scan_eblks(struct mtdswap_dev *d)
{
	int status;
	unsigned int i, idx;
	struct swap_eb *eb;

	for (i = 0; i < d->eblks; i++) {
		eb = d->eb_data + i;

		status = mtdswap_read_markers(d, eb);
		if (status < 0)
			eb->flags |= EBLOCK_READERR;
		else if (status == MTDSWAP_SCANNED_BAD) {
			eb->flags |= EBLOCK_BAD;
			continue;
		}

		switch (status) {
		case MTDSWAP_SCANNED_CLEAN:
			idx = MTDSWAP_CLEAN;
			break;
		case MTDSWAP_SCANNED_DIRTY:
		case MTDSWAP_SCANNED_BITFLIP:
			idx = MTDSWAP_DIRTY;
			break;
		default:
			idx = MTDSWAP_FAILING;
		}

		eb->flags |= (idx << EBLOCK_IDX_SHIFT);
	}

	mtdswap_check_counts(d);

	for (i = 0; i < d->eblks; i++) {
		eb = d->eb_data + i;

		if (eb->flags & EBLOCK_BAD)
			continue;

		idx = eb->flags >> EBLOCK_IDX_SHIFT;
		mtdswap_rb_add(d, eb, idx);
	}
}

/*
 * Place eblk into a tree corresponding to its number of active blocks
 * it contains.
 */
static void mtdswap_store_eb(struct mtdswap_dev *d, struct swap_eb *eb)
{
	unsigned int weight = eb->active_count;
	unsigned int maxweight = d->pages_per_eblk;

	if (eb == d->curr_write)
		return;

	if (eb->flags & EBLOCK_BITFLIP)
		mtdswap_rb_add(d, eb, MTDSWAP_BITFLIP);
	else if (eb->flags & (EBLOCK_READERR | EBLOCK_FAILED))
		mtdswap_rb_add(d, eb, MTDSWAP_FAILING);
	if (weight == maxweight)
		mtdswap_rb_add(d, eb, MTDSWAP_USED);
	else if (weight == 0)
		mtdswap_rb_add(d, eb, MTDSWAP_DIRTY);
	else if (weight > (maxweight/2))
		mtdswap_rb_add(d, eb, MTDSWAP_LOWFRAG);
	else
		mtdswap_rb_add(d, eb, MTDSWAP_HIFRAG);
}


static void mtdswap_erase_callback(struct erase_info *done)
{
	wait_queue_head_t *wait_q = (wait_queue_head_t *)done->priv;
	wake_up(wait_q);
}

static int mtdswap_erase_block(struct mtdswap_dev *d, struct swap_eb *eb)
{
	struct mtd_info *mtd = d->mtd;
	struct erase_info erase;
	wait_queue_head_t wq;
	unsigned int retries = 0;
	int ret;

	eb->erase_count++;
	if (eb->erase_count > d->max_erase_count)
		d->max_erase_count = eb->erase_count;

retry:
	init_waitqueue_head(&wq);
	memset(&erase, 0, sizeof(struct erase_info));

	erase.mtd	= mtd;
	erase.callback	= mtdswap_erase_callback;
	erase.addr	= mtdswap_eb_offset(d, eb);
	erase.len	= mtd->erasesize;
	erase.priv	= (u_long)&wq;

	ret = mtd_erase(mtd, &erase);
	if (ret) {
		if (retries++ < MTDSWAP_ERASE_RETRIES) {
			dev_warn(d->dev,
				"erase of erase block %#llx on %s failed",
				erase.addr, mtd->name);
			yield();
			goto retry;
		}

		dev_err(d->dev, "Cannot erase erase block %#llx on %s\n",
			erase.addr, mtd->name);

		mtdswap_handle_badblock(d, eb);
		return -EIO;
	}

	ret = wait_event_interruptible(wq, erase.state == MTD_ERASE_DONE ||
					   erase.state == MTD_ERASE_FAILED);
	if (ret) {
		dev_err(d->dev, "Interrupted erase block %#llx erasure on %s\n",
			erase.addr, mtd->name);
		return -EINTR;
	}

	if (erase.state == MTD_ERASE_FAILED) {
		if (retries++ < MTDSWAP_ERASE_RETRIES) {
			dev_warn(d->dev,
				"erase of erase block %#llx on %s failed",
				erase.addr, mtd->name);
			yield();
			goto retry;
		}

		mtdswap_handle_badblock(d, eb);
		return -EIO;
	}

	return 0;
}

static int mtdswap_map_free_block(struct mtdswap_dev *d, unsigned int page,
				unsigned int *block)
{
	int ret;
	struct swap_eb *old_eb = d->curr_write;
	struct rb_root *clean_root;
	struct swap_eb *eb;

	if (old_eb == NULL || d->curr_write_pos >= d->pages_per_eblk) {
		do {
			if (TREE_EMPTY(d, CLEAN))
				return -ENOSPC;

			clean_root = TREE_ROOT(d, CLEAN);
			eb = rb_entry(rb_first(clean_root), struct swap_eb, rb);
			rb_erase(&eb->rb, clean_root);
			eb->root = NULL;
			TREE_COUNT(d, CLEAN)--;

			ret = mtdswap_write_marker(d, eb, MTDSWAP_TYPE_DIRTY);
		} while (ret == -EIO || mtd_is_eccerr(ret));

		if (ret)
			return ret;

		d->curr_write_pos = 0;
		d->curr_write = eb;
		if (old_eb)
			mtdswap_store_eb(d, old_eb);
	}

	*block = (d->curr_write - d->eb_data) * d->pages_per_eblk +
		d->curr_write_pos;

	d->curr_write->active_count++;
	d->revmap[*block] = page;
	d->curr_write_pos++;

	return 0;
}

static unsigned int mtdswap_free_page_cnt(struct mtdswap_dev *d)
{
	return TREE_COUNT(d, CLEAN) * d->pages_per_eblk +
		d->pages_per_eblk - d->curr_write_pos;
}

static unsigned int mtdswap_enough_free_pages(struct mtdswap_dev *d)
{
	return mtdswap_free_page_cnt(d) > d->pages_per_eblk;
}

static int mtdswap_write_block(struct mtdswap_dev *d, char *buf,
			unsigned int page, unsigned int *bp, int gc_context)
{
	struct mtd_info *mtd = d->mtd;
	struct swap_eb *eb;
	size_t retlen;
	loff_t writepos;
	int ret;

retry:
	if (!gc_context)
		while (!mtdswap_enough_free_pages(d))
			if (mtdswap_gc(d, 0) > 0)
				return -ENOSPC;

	ret = mtdswap_map_free_block(d, page, bp);
	eb = d->eb_data + (*bp / d->pages_per_eblk);

	if (ret == -EIO || mtd_is_eccerr(ret)) {
		d->curr_write = NULL;
		eb->active_count--;
		d->revmap[*bp] = PAGE_UNDEF;
		goto retry;
	}

	if (ret < 0)
		return ret;

	writepos = (loff_t)*bp << PAGE_SHIFT;
	ret =  mtd_write(mtd, writepos, PAGE_SIZE, &retlen, buf);
	if (ret == -EIO || mtd_is_eccerr(ret)) {
		d->curr_write_pos--;
		eb->active_count--;
		d->revmap[*bp] = PAGE_UNDEF;
		mtdswap_handle_write_error(d, eb);
		goto retry;
	}

	if (ret < 0) {
		dev_err(d->dev, "Write to MTD device failed: %d (%zd written)",
			ret, retlen);
		goto err;
	}

	if (retlen != PAGE_SIZE) {
		dev_err(d->dev, "Short write to MTD device: %zd written",
			retlen);
		ret = -EIO;
		goto err;
	}

	return ret;

err:
	d->curr_write_pos--;
	eb->active_count--;
	d->revmap[*bp] = PAGE_UNDEF;

	return ret;
}

static int mtdswap_move_block(struct mtdswap_dev *d, unsigned int oldblock,
		unsigned int *newblock)
{
	struct mtd_info *mtd = d->mtd;
	struct swap_eb *eb, *oldeb;
	int ret;
	size_t retlen;
	unsigned int page, retries;
	loff_t readpos;

	page = d->revmap[oldblock];
	readpos = (loff_t) oldblock << PAGE_SHIFT;
	retries = 0;

retry:
	ret = mtd_read(mtd, readpos, PAGE_SIZE, &retlen, d->page_buf);

	if (ret < 0 && !mtd_is_bitflip(ret)) {
		oldeb = d->eb_data + oldblock / d->pages_per_eblk;
		oldeb->flags |= EBLOCK_READERR;

		dev_err(d->dev, "Read Error: %d (block %u)\n", ret,
			oldblock);
		retries++;
		if (retries < MTDSWAP_IO_RETRIES)
			goto retry;

		goto read_error;
	}

	if (retlen != PAGE_SIZE) {
		dev_err(d->dev, "Short read: %zd (block %u)\n", retlen,
		       oldblock);
		ret = -EIO;
		goto read_error;
	}

	ret = mtdswap_write_block(d, d->page_buf, page, newblock, 1);
	if (ret < 0) {
		d->page_data[page] = BLOCK_ERROR;
		dev_err(d->dev, "Write error: %d\n", ret);
		return ret;
	}

	eb = d->eb_data + *newblock / d->pages_per_eblk;
	d->page_data[page] = *newblock;
	d->revmap[oldblock] = PAGE_UNDEF;
	eb = d->eb_data + oldblock / d->pages_per_eblk;
	eb->active_count--;

	return 0;

read_error:
	d->page_data[page] = BLOCK_ERROR;
	d->revmap[oldblock] = PAGE_UNDEF;
	return ret;
}

static int mtdswap_gc_eblock(struct mtdswap_dev *d, struct swap_eb *eb)
{
	unsigned int i, block, eblk_base, newblock;
	int ret, errcode;

	errcode = 0;
	eblk_base = (eb - d->eb_data) * d->pages_per_eblk;

	for (i = 0; i < d->pages_per_eblk; i++) {
		if (d->spare_eblks < MIN_SPARE_EBLOCKS)
			return -ENOSPC;

		block = eblk_base + i;
		if (d->revmap[block] == PAGE_UNDEF)
			continue;

		ret = mtdswap_move_block(d, block, &newblock);
		if (ret < 0 && !errcode)
			errcode = ret;
	}

	return errcode;
}

static int __mtdswap_choose_gc_tree(struct mtdswap_dev *d)
{
	int idx, stopat;

	if (TREE_COUNT(d, CLEAN) < LOW_FRAG_GC_THRESHOLD)
		stopat = MTDSWAP_LOWFRAG;
	else
		stopat = MTDSWAP_HIFRAG;

	for (idx = MTDSWAP_BITFLIP; idx >= stopat; idx--)
		if (d->trees[idx].root.rb_node != NULL)
			return idx;

	return -1;
}

static int mtdswap_wlfreq(unsigned int maxdiff)
{
	unsigned int h, x, y, dist, base;

	/*
	 * Calculate linear ramp down from f1 to f2 when maxdiff goes from
	 * MAX_ERASE_DIFF to MAX_ERASE_DIFF + COLLECT_NONDIRTY_BASE.  Similar
	 * to triangle with height f1 - f1 and width COLLECT_NONDIRTY_BASE.
	 */

	dist = maxdiff - MAX_ERASE_DIFF;
	if (dist > COLLECT_NONDIRTY_BASE)
		dist = COLLECT_NONDIRTY_BASE;

	/*
	 * Modelling the slop as right angular triangle with base
	 * COLLECT_NONDIRTY_BASE and height freq1 - freq2. The ratio y/x is
	 * equal to the ratio h/base.
	 */
	h = COLLECT_NONDIRTY_FREQ1 - COLLECT_NONDIRTY_FREQ2;
	base = COLLECT_NONDIRTY_BASE;

	x = dist - base;
	y = (x * h + base / 2) / base;

	return COLLECT_NONDIRTY_FREQ2 + y;
}

static int mtdswap_choose_wl_tree(struct mtdswap_dev *d)
{
	static unsigned int pick_cnt;
	unsigned int i, idx = -1, wear, max;
	struct rb_root *root;

	max = 0;
	for (i = 0; i <= MTDSWAP_DIRTY; i++) {
		root = &d->trees[i].root;
		if (root->rb_node == NULL)
			continue;

		wear = d->max_erase_count - MTDSWAP_ECNT_MIN(root);
		if (wear > max) {
			max = wear;
			idx = i;
		}
	}

	if (max > MAX_ERASE_DIFF && pick_cnt >= mtdswap_wlfreq(max) - 1) {
		pick_cnt = 0;
		return idx;
	}

	pick_cnt++;
	return -1;
}

static int mtdswap_choose_gc_tree(struct mtdswap_dev *d,
				unsigned int background)
{
	int idx;

	if (TREE_NONEMPTY(d, FAILING) &&
		(background || (TREE_EMPTY(d, CLEAN) && TREE_EMPTY(d, DIRTY))))
		return MTDSWAP_FAILING;

	idx = mtdswap_choose_wl_tree(d);
	if (idx >= MTDSWAP_CLEAN)
		return idx;

	return __mtdswap_choose_gc_tree(d);
}

static struct swap_eb *mtdswap_pick_gc_eblk(struct mtdswap_dev *d,
					unsigned int background)
{
	struct rb_root *rp = NULL;
	struct swap_eb *eb = NULL;
	int idx;

	if (background && TREE_COUNT(d, CLEAN) > CLEAN_BLOCK_THRESHOLD &&
		TREE_EMPTY(d, DIRTY) && TREE_EMPTY(d, FAILING))
		return NULL;

	idx = mtdswap_choose_gc_tree(d, background);
	if (idx < 0)
		return NULL;

	rp = &d->trees[idx].root;
	eb = rb_entry(rb_first(rp), struct swap_eb, rb);

	rb_erase(&eb->rb, rp);
	eb->root = NULL;
	d->trees[idx].count--;
	return eb;
}

static unsigned int mtdswap_test_patt(unsigned int i)
{
	return i % 2 ? 0x55555555 : 0xAAAAAAAA;
}

static unsigned int mtdswap_eblk_passes(struct mtdswap_dev *d,
					struct swap_eb *eb)
{
	struct mtd_info *mtd = d->mtd;
	unsigned int test, i, j, patt, mtd_pages;
	loff_t base, pos;
	unsigned int *p1 = (unsigned int *)d->page_buf;
	unsigned char *p2 = (unsigned char *)d->oob_buf;
	struct mtd_oob_ops ops;
	int ret;

	ops.mode = MTD_OPS_AUTO_OOB;
	ops.len = mtd->writesize;
	ops.ooblen = mtd->oobavail;
	ops.ooboffs = 0;
	ops.datbuf = d->page_buf;
	ops.oobbuf = d->oob_buf;
	base = mtdswap_eb_offset(d, eb);
	mtd_pages = d->pages_per_eblk * PAGE_SIZE / mtd->writesize;

	for (test = 0; test < 2; test++) {
		pos = base;
		for (i = 0; i < mtd_pages; i++) {
			patt = mtdswap_test_patt(test + i);
			memset(d->page_buf, patt, mtd->writesize);
			memset(d->oob_buf, patt, mtd->oobavail);
			ret = mtd_write_oob(mtd, pos, &ops);
			if (ret)
				goto error;

			pos += mtd->writesize;
		}

		pos = base;
		for (i = 0; i < mtd_pages; i++) {
			ret = mtd_read_oob(mtd, pos, &ops);
			if (ret)
				goto error;

			patt = mtdswap_test_patt(test + i);
			for (j = 0; j < mtd->writesize/sizeof(int); j++)
				if (p1[j] != patt)
					goto error;

			for (j = 0; j < mtd->oobavail; j++)
				if (p2[j] != (unsigned char)patt)
					goto error;

			pos += mtd->writesize;
		}

		ret = mtdswap_erase_block(d, eb);
		if (ret)
			goto error;
	}

	eb->flags &= ~EBLOCK_READERR;
	return 1;

error:
	mtdswap_handle_badblock(d, eb);
	return 0;
}

static int mtdswap_gc(struct mtdswap_dev *d, unsigned int background)
{
	struct swap_eb *eb;
	int ret;

	if (d->spare_eblks < MIN_SPARE_EBLOCKS)
		return 1;

	eb = mtdswap_pick_gc_eblk(d, background);
	if (!eb)
		return 1;

	ret = mtdswap_gc_eblock(d, eb);
	if (ret == -ENOSPC)
		return 1;

	if (eb->flags & EBLOCK_FAILED) {
		mtdswap_handle_badblock(d, eb);
		return 0;
	}

	eb->flags &= ~EBLOCK_BITFLIP;
	ret = mtdswap_erase_block(d, eb);
	if ((eb->flags & EBLOCK_READERR) &&
		(ret || !mtdswap_eblk_passes(d, eb)))
		return 0;

	if (ret == 0)
		ret = mtdswap_write_marker(d, eb, MTDSWAP_TYPE_CLEAN);

	if (ret == 0)
		mtdswap_rb_add(d, eb, MTDSWAP_CLEAN);
	else if (ret != -EIO && !mtd_is_eccerr(ret))
		mtdswap_rb_add(d, eb, MTDSWAP_DIRTY);

	return 0;
}

static void mtdswap_background(struct mtd_blktrans_dev *dev)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);
	int ret;

	while (1) {
		ret = mtdswap_gc(d, 1);
		if (ret || mtd_blktrans_cease_background(dev))
			return;
	}
}

static void mtdswap_cleanup(struct mtdswap_dev *d)
{
	vfree(d->eb_data);
	vfree(d->revmap);
	vfree(d->page_data);
	kfree(d->oob_buf);
	kfree(d->page_buf);
}

static int mtdswap_flush(struct mtd_blktrans_dev *dev)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);

	mtd_sync(d->mtd);
	return 0;
}

static unsigned int mtdswap_badblocks(struct mtd_info *mtd, uint64_t size)
{
	loff_t offset;
	unsigned int badcnt;

	badcnt = 0;

	if (mtd_can_have_bb(mtd))
		for (offset = 0; offset < size; offset += mtd->erasesize)
			if (mtd_block_isbad(mtd, offset))
				badcnt++;

	return badcnt;
}

static int mtdswap_writesect(struct mtd_blktrans_dev *dev,
			unsigned long page, char *buf)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);
	unsigned int newblock, mapped;
	struct swap_eb *eb;
	int ret;

	d->sect_write_count++;

	if (d->spare_eblks < MIN_SPARE_EBLOCKS)
		return -ENOSPC;

	if (header) {
		/* Ignore writes to the header page */
		if (unlikely(page == 0))
			return 0;

		page--;
	}

	mapped = d->page_data[page];
	if (mapped <= BLOCK_MAX) {
		eb = d->eb_data + (mapped / d->pages_per_eblk);
		eb->active_count--;
		mtdswap_store_eb(d, eb);
		d->page_data[page] = BLOCK_UNDEF;
		d->revmap[mapped] = PAGE_UNDEF;
	}

	ret = mtdswap_write_block(d, buf, page, &newblock, 0);
	d->mtd_write_count++;

	if (ret < 0)
		return ret;

	eb = d->eb_data + (newblock / d->pages_per_eblk);
	d->page_data[page] = newblock;

	return 0;
}

/* Provide a dummy swap header for the kernel */
static int mtdswap_auto_header(struct mtdswap_dev *d, char *buf)
{
	union swap_header *hd = (union swap_header *)(buf);

	memset(buf, 0, PAGE_SIZE - 10);

	hd->info.version = 1;
	hd->info.last_page = d->mbd_dev->size - 1;
	hd->info.nr_badpages = 0;

	memcpy(buf + PAGE_SIZE - 10, "SWAPSPACE2", 10);

	return 0;
}

static int mtdswap_readsect(struct mtd_blktrans_dev *dev,
			unsigned long page, char *buf)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);
	struct mtd_info *mtd = d->mtd;
	unsigned int realblock, retries;
	loff_t readpos;
	struct swap_eb *eb;
	size_t retlen;
	int ret;

	d->sect_read_count++;

	if (header) {
		if (unlikely(page == 0))
			return mtdswap_auto_header(d, buf);

		page--;
	}

	realblock = d->page_data[page];
	if (realblock > BLOCK_MAX) {
		memset(buf, 0x0, PAGE_SIZE);
		if (realblock == BLOCK_UNDEF)
			return 0;
		else
			return -EIO;
	}

	eb = d->eb_data + (realblock / d->pages_per_eblk);
	BUG_ON(d->revmap[realblock] == PAGE_UNDEF);

	readpos = (loff_t)realblock << PAGE_SHIFT;
	retries = 0;

retry:
	ret = mtd_read(mtd, readpos, PAGE_SIZE, &retlen, buf);

	d->mtd_read_count++;
	if (mtd_is_bitflip(ret)) {
		eb->flags |= EBLOCK_BITFLIP;
		mtdswap_rb_add(d, eb, MTDSWAP_BITFLIP);
		ret = 0;
	}

	if (ret < 0) {
		dev_err(d->dev, "Read error %d\n", ret);
		eb->flags |= EBLOCK_READERR;
		mtdswap_rb_add(d, eb, MTDSWAP_FAILING);
		retries++;
		if (retries < MTDSWAP_IO_RETRIES)
			goto retry;

		return ret;
	}

	if (retlen != PAGE_SIZE) {
		dev_err(d->dev, "Short read %zd\n", retlen);
		return -EIO;
	}

	return 0;
}

static int mtdswap_discard(struct mtd_blktrans_dev *dev, unsigned long first,
			unsigned nr_pages)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);
	unsigned long page;
	struct swap_eb *eb;
	unsigned int mapped;

	d->discard_count++;

	for (page = first; page < first + nr_pages; page++) {
		mapped = d->page_data[page];
		if (mapped <= BLOCK_MAX) {
			eb = d->eb_data + (mapped / d->pages_per_eblk);
			eb->active_count--;
			mtdswap_store_eb(d, eb);
			d->page_data[page] = BLOCK_UNDEF;
			d->revmap[mapped] = PAGE_UNDEF;
			d->discard_page_count++;
		} else if (mapped == BLOCK_ERROR) {
			d->page_data[page] = BLOCK_UNDEF;
			d->discard_page_count++;
		}
	}

	return 0;
}

static int mtdswap_show(struct seq_file *s, void *data)
{
	struct mtdswap_dev *d = (struct mtdswap_dev *) s->private;
	unsigned long sum;
	unsigned int count[MTDSWAP_TREE_CNT];
	unsigned int min[MTDSWAP_TREE_CNT];
	unsigned int max[MTDSWAP_TREE_CNT];
	unsigned int i, cw = 0, cwp = 0, cwecount = 0, bb_cnt, mapped, pages;
	uint64_t use_size;
	static const char * const name[] = {
		"clean", "used", "low", "high", "dirty", "bitflip", "failing"
	};

	mutex_lock(&d->mbd_dev->lock);

	for (i = 0; i < MTDSWAP_TREE_CNT; i++) {
		struct rb_root *root = &d->trees[i].root;

		if (root->rb_node) {
			count[i] = d->trees[i].count;
			min[i] = MTDSWAP_ECNT_MIN(root);
			max[i] = MTDSWAP_ECNT_MAX(root);
		} else
			count[i] = 0;
	}

	if (d->curr_write) {
		cw = 1;
		cwp = d->curr_write_pos;
		cwecount = d->curr_write->erase_count;
	}

	sum = 0;
	for (i = 0; i < d->eblks; i++)
		sum += d->eb_data[i].erase_count;

	use_size = (uint64_t)d->eblks * d->mtd->erasesize;
	bb_cnt = mtdswap_badblocks(d->mtd, use_size);

	mapped = 0;
	pages = d->mbd_dev->size;
	for (i = 0; i < pages; i++)
		if (d->page_data[i] != BLOCK_UNDEF)
			mapped++;

	mutex_unlock(&d->mbd_dev->lock);

	for (i = 0; i < MTDSWAP_TREE_CNT; i++) {
		if (!count[i])
			continue;

		if (min[i] != max[i])
			seq_printf(s, "%s:\t%5d erase blocks, erased min %d, "
				"max %d times\n",
				name[i], count[i], min[i], max[i]);
		else
			seq_printf(s, "%s:\t%5d erase blocks, all erased %d "
				"times\n", name[i], count[i], min[i]);
	}

	if (bb_cnt)
		seq_printf(s, "bad:\t%5u erase blocks\n", bb_cnt);

	if (cw)
		seq_printf(s, "current erase block: %u pages used, %u free, "
			"erased %u times\n",
			cwp, d->pages_per_eblk - cwp, cwecount);

	seq_printf(s, "total erasures: %lu\n", sum);

	seq_puts(s, "\n");

	seq_printf(s, "mtdswap_readsect count: %llu\n", d->sect_read_count);
	seq_printf(s, "mtdswap_writesect count: %llu\n", d->sect_write_count);
	seq_printf(s, "mtdswap_discard count: %llu\n", d->discard_count);
	seq_printf(s, "mtd read count: %llu\n", d->mtd_read_count);
	seq_printf(s, "mtd write count: %llu\n", d->mtd_write_count);
	seq_printf(s, "discarded pages count: %llu\n", d->discard_page_count);

	seq_puts(s, "\n");
	seq_printf(s, "total pages: %u\n", pages);
	seq_printf(s, "pages mapped: %u\n", mapped);

	return 0;
}

static int mtdswap_open(struct inode *inode, struct file *file)
{
	return single_open(file, mtdswap_show, inode->i_private);
}

static const struct file_operations mtdswap_fops = {
	.open		= mtdswap_open,
	.read		= seq_read,
	.llseek		= seq_lseek,
	.release	= single_release,
};

static int mtdswap_add_debugfs(struct mtdswap_dev *d)
{
	struct dentry *root = d->mtd->dbg.dfs_dir;
	struct dentry *dent;

	if (!IS_ENABLED(CONFIG_DEBUG_FS))
		return 0;

	if (IS_ERR_OR_NULL(root))
		return -1;

	dent = debugfs_create_file("mtdswap_stats", S_IRUSR, root, d,
				&mtdswap_fops);
	if (!dent) {
		dev_err(d->dev, "debugfs_create_file failed\n");
		return -1;
	}

	return 0;
}

static int mtdswap_init(struct mtdswap_dev *d, unsigned int eblocks,
			unsigned int spare_cnt)
{
	struct mtd_info *mtd = d->mbd_dev->mtd;
	unsigned int i, eblk_bytes, pages, blocks;
	int ret = -ENOMEM;

	d->mtd = mtd;
	d->eblks = eblocks;
	d->spare_eblks = spare_cnt;
	d->pages_per_eblk = mtd->erasesize >> PAGE_SHIFT;

	pages = d->mbd_dev->size;
	blocks = eblocks * d->pages_per_eblk;

	for (i = 0; i < MTDSWAP_TREE_CNT; i++)
		d->trees[i].root = RB_ROOT;

	d->page_data = vmalloc(sizeof(int)*pages);
	if (!d->page_data)
		goto page_data_fail;

	d->revmap = vmalloc(sizeof(int)*blocks);
	if (!d->revmap)
		goto revmap_fail;

	eblk_bytes = sizeof(struct swap_eb)*d->eblks;
	d->eb_data = vzalloc(eblk_bytes);
	if (!d->eb_data)
		goto eb_data_fail;

	for (i = 0; i < pages; i++)
		d->page_data[i] = BLOCK_UNDEF;

	for (i = 0; i < blocks; i++)
		d->revmap[i] = PAGE_UNDEF;

	d->page_buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
	if (!d->page_buf)
		goto page_buf_fail;

	d->oob_buf = kmalloc(2 * mtd->oobavail, GFP_KERNEL);
	if (!d->oob_buf)
		goto oob_buf_fail;

	mtdswap_scan_eblks(d);

	return 0;

oob_buf_fail:
	kfree(d->page_buf);
page_buf_fail:
	vfree(d->eb_data);
eb_data_fail:
	vfree(d->revmap);
revmap_fail:
	vfree(d->page_data);
page_data_fail:
	printk(KERN_ERR "%s: init failed (%d)\n", MTDSWAP_PREFIX, ret);
	return ret;
}

static void mtdswap_add_mtd(struct mtd_blktrans_ops *tr, struct mtd_info *mtd)
{
	struct mtdswap_dev *d;
	struct mtd_blktrans_dev *mbd_dev;
	char *parts;
	char *this_opt;
	unsigned long part;
	unsigned int eblocks, eavailable, bad_blocks, spare_cnt;
	uint64_t swap_size, use_size, size_limit;
	int ret;

	parts = &partitions[0];
	if (!*parts)
		return;

	while ((this_opt = strsep(&parts, ",")) != NULL) {
		if (kstrtoul(this_opt, 0, &part) < 0)
			return;

		if (mtd->index == part)
			break;
	}

	if (mtd->index != part)
		return;

	if (mtd->erasesize < PAGE_SIZE || mtd->erasesize % PAGE_SIZE) {
		printk(KERN_ERR "%s: Erase size %u not multiple of PAGE_SIZE "
			"%lu\n", MTDSWAP_PREFIX, mtd->erasesize, PAGE_SIZE);
		return;
	}

	if (PAGE_SIZE % mtd->writesize || mtd->writesize > PAGE_SIZE) {
		printk(KERN_ERR "%s: PAGE_SIZE %lu not multiple of write size"
			" %u\n", MTDSWAP_PREFIX, PAGE_SIZE, mtd->writesize);
		return;
	}

	if (!mtd->oobsize || mtd->oobavail < MTDSWAP_OOBSIZE) {
		printk(KERN_ERR "%s: Not enough free bytes in OOB, "
			"%d available, %zu needed.\n",
			MTDSWAP_PREFIX, mtd->oobavail, MTDSWAP_OOBSIZE);
		return;
	}

	if (spare_eblocks > 100)
		spare_eblocks = 100;

	use_size = mtd->size;
	size_limit = (uint64_t) BLOCK_MAX * PAGE_SIZE;

	if (mtd->size > size_limit) {
		printk(KERN_WARNING "%s: Device too large. Limiting size to "
			"%llu bytes\n", MTDSWAP_PREFIX, size_limit);
		use_size = size_limit;
	}

	eblocks = mtd_div_by_eb(use_size, mtd);
	use_size = (uint64_t)eblocks * mtd->erasesize;
	bad_blocks = mtdswap_badblocks(mtd, use_size);
	eavailable = eblocks - bad_blocks;

	if (eavailable < MIN_ERASE_BLOCKS) {
		printk(KERN_ERR "%s: Not enough erase blocks. %u available, "
			"%d needed\n", MTDSWAP_PREFIX, eavailable,
			MIN_ERASE_BLOCKS);
		return;
	}

	spare_cnt = div_u64((uint64_t)eavailable * spare_eblocks, 100);

	if (spare_cnt < MIN_SPARE_EBLOCKS)
		spare_cnt = MIN_SPARE_EBLOCKS;

	if (spare_cnt > eavailable - 1)
		spare_cnt = eavailable - 1;

	swap_size = (uint64_t)(eavailable - spare_cnt) * mtd->erasesize +
		(header ? PAGE_SIZE : 0);

	printk(KERN_INFO "%s: Enabling MTD swap on device %lu, size %llu KB, "
		"%u spare, %u bad blocks\n",
		MTDSWAP_PREFIX, part, swap_size / 1024, spare_cnt, bad_blocks);

	d = kzalloc(sizeof(struct mtdswap_dev), GFP_KERNEL);
	if (!d)
		return;

	mbd_dev = kzalloc(sizeof(struct mtd_blktrans_dev), GFP_KERNEL);
	if (!mbd_dev) {
		kfree(d);
		return;
	}

	d->mbd_dev = mbd_dev;
	mbd_dev->priv = d;

	mbd_dev->mtd = mtd;
	mbd_dev->devnum = mtd->index;
	mbd_dev->size = swap_size >> PAGE_SHIFT;
	mbd_dev->tr = tr;

	if (!(mtd->flags & MTD_WRITEABLE))
		mbd_dev->readonly = 1;

	if (mtdswap_init(d, eblocks, spare_cnt) < 0)
		goto init_failed;

	if (add_mtd_blktrans_dev(mbd_dev) < 0)
		goto cleanup;

	d->dev = disk_to_dev(mbd_dev->disk);

	ret = mtdswap_add_debugfs(d);
	if (ret < 0)
		goto debugfs_failed;

	return;

debugfs_failed:
	del_mtd_blktrans_dev(mbd_dev);

cleanup:
	mtdswap_cleanup(d);

init_failed:
	kfree(mbd_dev);
	kfree(d);
}

static void mtdswap_remove_dev(struct mtd_blktrans_dev *dev)
{
	struct mtdswap_dev *d = MTDSWAP_MBD_TO_MTDSWAP(dev);

	del_mtd_blktrans_dev(dev);
	mtdswap_cleanup(d);
	kfree(d);
}

static struct mtd_blktrans_ops mtdswap_ops = {
	.name		= "mtdswap",
	.major		= 0,
	.part_bits	= 0,
	.blksize	= PAGE_SIZE,
	.flush		= mtdswap_flush,
	.readsect	= mtdswap_readsect,
	.writesect	= mtdswap_writesect,
	.discard	= mtdswap_discard,
	.background	= mtdswap_background,
	.add_mtd	= mtdswap_add_mtd,
	.remove_dev	= mtdswap_remove_dev,
	.owner		= THIS_MODULE,
};

static int __init mtdswap_modinit(void)
{
	return register_mtd_blktrans(&mtdswap_ops);
}

static void __exit mtdswap_modexit(void)
{
	deregister_mtd_blktrans(&mtdswap_ops);
}

module_init(mtdswap_modinit);
module_exit(mtdswap_modexit);


MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jarkko Lavinen <jarkko.lavinen@nokia.com>");
MODULE_DESCRIPTION("Block device access to an MTD suitable for using as "
		"swap space");