Re: [PATCH v2 1/2] btrfs: do not try compression for data reloc inodes

On Mon, Jun 22, 2026 at 11:56 PM Qu Wenruo <[email protected]> wrote:
>
> [BUG]
> There is a syzbot report that the check inside get_new_location()
> triggered:
>
>  BTRFS info (device loop0): found 31 extents, stage: move data extents
>  BTRFS info (device loop0): leaf 8908800 gen 16 total ptrs 28 free space 1676 owner 18446744073709551607
>         item 0 key (256 INODE_ITEM 0) itemoff 3835 itemsize 160
>                 inode generation 5 transid 0 size 0 nbytes 0
>                 block group 0 mode 40755 links 1 uid 0 gid 0
>                 rdev 0 sequence 0 flags 0x0
>                 atime 1669132761.0
>                 ctime 1669132761.0
>                 mtime 1669132761.0
>                 otime 0.0
>         item 1 key (256 INODE_REF 256) itemoff 3823 itemsize 12
>                 index 0 name_len 2
>         item 2 key (258 INODE_ITEM 0) itemoff 3663 itemsize 160
>                 inode generation 1 transid 16 size 733184 nbytes 106496
>                 block group 0 mode 100600 links 0 uid 0 gid 0
>                 rdev 0 sequence 24 flags 0x18
>         item 3 key (258 EXTENT_DATA 0) itemoff 3595 itemsize 68
>                 generation 16 type 0
>                 inline extent data size 47 ram_bytes 4096 compression 1
>  [...]
>         item 27 key (18446744073709551611 ORPHAN_ITEM 258) itemoff 2376 itemsize 0
>  BTRFS error (device loop0): unexpected non-zero offset in file extent item for data reloc inode 258 key offset 0 offset 9277520992061368337
>  ------------[ cut here ]------------
>  btrfs_abort_should_print_stack(__error)
>
> [CAUSE]
> The above dump tree shows the first file extent item is inlined, which
> should make no sense for data reloc inodes, as such inodes are just
> representing where the data extents are in the relocation destination chunk.
>
> However the relocation path is just preallocate space for each block,

"is just preallocate" is weird, more like "preallocates"

> then dirty them, cluster by cluster.
> It's possible to have a single block at the beginning of the block
> group, and no other block in the same cluster.
>
> Then relocation will preallocate a file extent for that block, dirty the first block.
> Then memory pressure forces the data reloc inode to be written back, before
> any other blocks being dirtied/allocated.
>
> Finally commit 3eaf5f082c4c ("btrfs: extract inlined creation into a dedicated
> delalloc helper") changed the timing of delalloc, before that commit we

This isn't about changing the timing but rather changing the order of
the rule set for inline/compression/nocow.
Saying timing is confusing.

> always try NOCOW first, so that dirtied block will be written back into
> the preallocated space.
>
> But with that commit, we always try inline first, and since compression
> is forced, we try compressing the first block, and then inline the
> compressed data, resulting in the above inlined file extent in data
> reloc tree.
>
> Then the check in get_new_location() will check the file offset, without
> checking if the file extent is inlined or not, resulting the above
> failure.
>
> [FIX]
> Do not allow compression for data reloc inodes in the first place.

I would add an explanation here about why disallowing compression
prevents the inline extent, as this is non-obvious and involves a
convoluted path.

So adding this new check in btrfs_inode_can_compress() makes
run_delalloc_inline() skip the compression `if` statement. Then, when
it calls can_cow_file_range_inline() that returns false because the
data size is >= sector size, causing run_delalloc_inline() to return
1, signaling to the caller, btrfs_run_delalloc_range(), it cannot
inline and must follow the nocow path.

Thanks.

>
> Reported-by: [email protected]
> Link: https://lore.kernel.org/linux-btrfs/[email protected]/
> Fixes: 3eaf5f082c4c ("btrfs: extract inlined creation into a dedicated delalloc helper")
> Cc: [email protected]
> Signed-off-by: Qu Wenruo <[email protected]>
> ---
>  fs/btrfs/btrfs_inode.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
> index d5d81f9546c3..fff72f6cc1e8 100644
> --- a/fs/btrfs/btrfs_inode.h
> +++ b/fs/btrfs/btrfs_inode.h
> @@ -476,6 +476,8 @@ static inline bool btrfs_inode_can_compress(const struct btrfs_inode *inode)
>         if (inode->flags & BTRFS_INODE_NODATACOW ||
>             inode->flags & BTRFS_INODE_NODATASUM)
>                 return false;
> +       if (btrfs_root_id(inode->root) == BTRFS_DATA_RELOC_TREE_OBJECTID)
> +               return false;
>         return true;
>  }
>
> --
> 2.54.0
>
>