Re: [PATCH] btrfs: only account delalloc bytes for regular file inodes in btrfs_getattr()

On Fri, Jun 26, 2026 at 7:52 AM Dave Chen <[email protected]> wrote:
>
> btrfs_getattr() unconditionally reads BTRFS_I(inode)->new_delalloc_bytes
> and adds it (sector-aligned) to stat->blocks for every inode type.
> However, new_delalloc_bytes lives in a union with last_dir_index_offset:
>
>     union {
>         u64 new_delalloc_bytes;     /* files only */
>         u64 last_dir_index_offset;  /* directories only */
>     };
>
> For a directory inode this memory holds last_dir_index_offset, which is
> set during directory logging (e.g. flush_dir_items_batch()) to the
> offset of the last logged BTRFS_DIR_INDEX_KEY.  That offset grows with
> the number of entries ever created in the directory (dir indexes are
> monotonic and never reused), so it can be arbitrarily large.
>
> As a result, after a directory has been logged (e.g. via an fsync that
> triggers directory logging), btrfs_getattr() reports inflated st_blocks
> for that directory.  The inflation is purely in-core and disappears
> after the inode is evicted and reloaded (btrfs_alloc_inode() zeroes the
> union), e.g. after a remount.
>
> Reproducer (on a btrfs filesystem):
>
>     D=/mnt/btrfs/d
>     mkdir -p $D
>     for i in $(seq 1 20000); do touch $D/f$i; done
>     sync                      # commit, push dir index high
>     touch $D/trigger          # dirty the dir in a new transaction
>     xfs_io -c fsync $D        # log the directory -> sets last_dir_index_offset
>     stat -c '%b' $D           # st_blocks is now inflated (e.g. 40)
>     # umount + mount -> st_blocks drops back to the correct value
>
> The evict path already knows this union is type-dependent and guards the
> corresponding WARN_ON with !S_ISDIR() in btrfs_destroy_inode(); only
> btrfs_getattr() was missing the equivalent check.
>
> Only read new_delalloc_bytes for regular files, which are the only
> inodes that ever set it.
>
> Signed-off-by: Dave Chen <[email protected]>

Reviewed-by: Filipe Manana <[email protected]>

Will add it soon to github's for-next branch, thanks.

> ---
>  fs/btrfs/inode.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 272598f6ae77..b446c3014b24 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -8068,7 +8068,8 @@ static int btrfs_getattr(struct mnt_idmap *idmap,
>         stat->result_mask |= STATX_SUBVOL;
>
>         spin_lock(&BTRFS_I(inode)->lock);
> -       delalloc_bytes = BTRFS_I(inode)->new_delalloc_bytes;
> +       delalloc_bytes = S_ISREG(inode->i_mode) ?
> +                        BTRFS_I(inode)->new_delalloc_bytes : 0;
>         inode_bytes = inode_get_bytes(inode);
>         spin_unlock(&BTRFS_I(inode)->lock);
>         stat->blocks = (ALIGN(inode_bytes, blocksize) +
> --
> 2.43.0
>
>
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