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btrfs: backref: introduce the skeleton of btrfs_backref_iter 

Due to the complex nature of btrfs extent tree, when we want to iterate
all backrefs of one extent, this involves quite a lot of work, like
searching the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
backrefs.

Normally this would result in a complex code, something like:

  btrfs_search_slot()
  /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
  while (1) {	/* Loop for extent tree items */
	while (ptr < end) { /* Loop for inlined items */
		/* Real work here */
	}
  next:
  	ret = btrfs_next_item()
	/* Ensure we're still at keyed item for specified bytenr */
  }

The idea of btrfs_backref_iter is to avoid such complex and hard to
read code structure, but something like the following:

  iter = btrfs_backref_iter_alloc();
  ret = btrfs_backref_iter_start(iter, bytenr);
  if (ret < 0)
	goto out;
  for (; ; ret = btrfs_backref_iter_next(iter)) {
	/* Real work here */
  }
  out:
  btrfs_backref_iter_free(iter);

This patch is just the skeleton + btrfs_backref_iter_start() code.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2020-02-13 14:11:04 +08:00 committed by David Sterba
parent 78d933c79c
commit a37f232b7b
2 changed files with 148 additions and 0 deletions
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110
fs/btrfs/backref.c
View File

@ -2295,3 +2295,113 @@ void free_ipath(struct inode_fs_paths *ipath)
kvfree(ipath->fspath);
kfree(ipath);
}
struct btrfs_backref_iter *btrfs_backref_iter_alloc(
struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
{
struct btrfs_backref_iter *ret;
ret = kzalloc(sizeof(*ret), gfp_flag);
if (!ret)
return NULL;
ret->path = btrfs_alloc_path();
if (!ret) {
kfree(ret);
return NULL;
}
/* Current backref iterator only supports iteration in commit root */
ret->path->search_commit_root = 1;
ret->path->skip_locking = 1;
ret->fs_info = fs_info;
return ret;
}
int btrfs_backref_iter_start(struct btrfs_backref_iter *iter, u64 bytenr)
{
struct btrfs_fs_info *fs_info = iter->fs_info;
struct btrfs_path *path = iter->path;
struct btrfs_extent_item *ei;
struct btrfs_key key;
int ret;
key.objectid = bytenr;
key.type = BTRFS_METADATA_ITEM_KEY;
key.offset = (u64)-1;
iter->bytenr = bytenr;
ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
if (ret < 0)
return ret;
if (ret == 0) {
ret = -EUCLEAN;
goto release;
}
if (path->slots[0] == 0) {
WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
ret = -EUCLEAN;
goto release;
}
path->slots[0]--;
btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
if ((key.type != BTRFS_EXTENT_ITEM_KEY &&
key.type != BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
ret = -ENOENT;
goto release;
}
memcpy(&iter->cur_key, &key, sizeof(key));
iter->item_ptr = (u32)btrfs_item_ptr_offset(path->nodes[0],
path->slots[0]);
iter->end_ptr = (u32)(iter->item_ptr +
btrfs_item_size_nr(path->nodes[0], path->slots[0]));
ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
struct btrfs_extent_item);
/*
* Only support iteration on tree backref yet.
*
* This is an extra precaution for non skinny-metadata, where
* EXTENT_ITEM is also used for tree blocks, that we can only use
* extent flags to determine if it's a tree block.
*/
if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
ret = -ENOTSUPP;
goto release;
}
iter->cur_ptr = (u32)(iter->item_ptr + sizeof(*ei));
/* If there is no inline backref, go search for keyed backref */
if (iter->cur_ptr >= iter->end_ptr) {
ret = btrfs_next_item(fs_info->extent_root, path);
/* No inline nor keyed ref */
if (ret > 0) {
ret = -ENOENT;
goto release;
}
if (ret < 0)
goto release;
btrfs_item_key_to_cpu(path->nodes[0], &iter->cur_key,
path->slots[0]);
if (iter->cur_key.objectid != bytenr ||
(iter->cur_key.type != BTRFS_SHARED_BLOCK_REF_KEY &&
iter->cur_key.type != BTRFS_TREE_BLOCK_REF_KEY)) {
ret = -ENOENT;
goto release;
}
iter->cur_ptr = (u32)btrfs_item_ptr_offset(path->nodes[0],
path->slots[0]);
iter->item_ptr = iter->cur_ptr;
iter->end_ptr = (u32)(iter->item_ptr + btrfs_item_size_nr(
path->nodes[0], path->slots[0]));
}
return 0;
release:
btrfs_backref_iter_release(iter);
return ret;
}

38
fs/btrfs/backref.h
View File

@ -78,4 +78,42 @@ struct prelim_ref {
u64 wanted_disk_byte;
};
/*
* Iterate backrefs of one extent.
*
* Now it only supports iteration of tree block in commit root.
*/
struct btrfs_backref_iter {
u64 bytenr;
struct btrfs_path *path;
struct btrfs_fs_info *fs_info;
struct btrfs_key cur_key;
u32 item_ptr;
u32 cur_ptr;
u32 end_ptr;
};
struct btrfs_backref_iter *btrfs_backref_iter_alloc(
struct btrfs_fs_info *fs_info, gfp_t gfp_flag);
static inline void btrfs_backref_iter_free(struct btrfs_backref_iter *iter)
{
if (!iter)
return;
btrfs_free_path(iter->path);
kfree(iter);
}
int btrfs_backref_iter_start(struct btrfs_backref_iter *iter, u64 bytenr);
static inline void btrfs_backref_iter_release(struct btrfs_backref_iter *iter)
{
iter->bytenr = 0;
iter->item_ptr = 0;
iter->cur_ptr = 0;
iter->end_ptr = 0;
btrfs_release_path(iter->path);
memset(&iter->cur_key, 0, sizeof(iter->cur_key));
}
#endif