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btrfs-progs: free-space-cache: Enhance free space cache free space check 

When we found free space difference between free space cache and block
group item, we just discard this free space cache.

Normally such difference is caused by btrfs_reserve_extent() called by
delalloc which is out of a transaction.
And since all btrfs_release_extent() is called with a transaction, under
heavy race free space cache can have less free space than block group
item.

Normally kernel will detect such difference and just discard that cache.

However we must be more careful if free space cache has more free space
cache, and if that happens, paried with above race one invalid free
space cache can be loaded into kernel.

So if we find any free space cache who has more free space then block
group item, we report it as an error other than ignoring it.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2018-03-08 15:02:31 +08:00 committed by David Sterba
parent fd5767a574
commit 4c7094e046
2 changed files with 27 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
check/main.c
View File

@ -5339,7 +5339,9 @@ static int check_space_cache(struct btrfs_root *root)
error += ret;
} else {
ret = load_free_space_cache(root->fs_info, cache);
if (!ret)
if (ret < 0)
error++;
if (ret <= 0)
continue;
}

30
free-space-cache.c
View File

@ -438,7 +438,8 @@ int load_free_space_cache(struct btrfs_fs_info *fs_info,
struct btrfs_path *path;
u64 used = btrfs_block_group_used(&block_group->item);
int ret = 0;
int matched;
u64 bg_free;
s64 diff;
path = btrfs_alloc_path();
if (!path)
@ -448,17 +449,32 @@ int load_free_space_cache(struct btrfs_fs_info *fs_info,
block_group->key.objectid);
btrfs_free_path(path);
matched = (ctl->free_space == (block_group->key.offset - used -
block_group->bytes_super));
if (ret == 1 && !matched) {
__btrfs_remove_free_space_cache(ctl);
bg_free = block_group->key.offset - used - block_group->bytes_super;
diff = ctl->free_space - bg_free;
if (ret == 1 && diff) {
fprintf(stderr,
"block group %llu has wrong amount of free space\n",
block_group->key.objectid);
"block group %llu has wrong amount of free space, free space cache has %llu block group has %llu\n",
block_group->key.objectid, ctl->free_space, bg_free);
__btrfs_remove_free_space_cache(ctl);
/*
* Due to btrfs_reserve_extent() can happen out of a
* transaction, but all btrfs_release_extent() happens inside
* a transaction, so under heavy race it's possible that free
* space cache has less free space, and both kernel just discard
* such cache. But if we find some case where free space cache
* has more free space, this means under certain case such
* cache can be loaded and cause double allocate.
*
* Detect such possibility here.
*/
if (diff > 0)
error(
"free space cache has more free space than block group item, this could leads to serious corruption, please contact btrfs developers");
ret = -1;
}
if (ret < 0) {
if (diff <= 0)
ret = 0;
fprintf(stderr,