Re: [PATCH] xfs: fix an off-by-one error in xreap_agextent_binval

[Dave Chinner <david@fromorbit.com>]

On Thu, Dec 14, 2023 at 01:38:45PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Overall, this function tries to find and invalidate all buffers for a
> given extent of space on the data device.  The inner for loop in this
> function tries to find all xfs_bufs for a given daddr.  The lengths of
> all possible cached buffers range from 1 fsblock to the largest needed
> to contain a 64k xattr value (~17fsb).  The scan is capped to avoid
> looking at anything buffer going past the given extent.
> 
> Unfortunately, the loop continuation test is wrong -- max_fsbs is the
> largest size we want to scan, not one past that.  Put another way, this
> loop is actually 1-indexed, not 0-indexed.  Therefore, the continuation
> test should use <=, not <.
> 
> As a result, online repairs of btree blocks fails to stale any buffers
> for btrees that are being torn down, which causes later assertions in
> the buffer cache when another thread creates a different-sized buffer.
> This happens in xfs/709 when allocating an inode cluster buffer:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 3346128 at fs/xfs/xfs_message.c:104 assfail+0x3a/0x40 [xfs]
>  CPU: 0 PID: 3346128 Comm: fsstress Not tainted 6.7.0-rc4-djwx #rc4
>  RIP: 0010:assfail+0x3a/0x40 [xfs]
>  Call Trace:
>   <TASK>
>   _xfs_buf_obj_cmp+0x4a/0x50
>   xfs_buf_get_map+0x191/0xba0
>   xfs_trans_get_buf_map+0x136/0x280
>   xfs_ialloc_inode_init+0x186/0x340
>   xfs_ialloc_ag_alloc+0x254/0x720
>   xfs_dialloc+0x21f/0x870
>   xfs_create_tmpfile+0x1a9/0x2f0
>   xfs_rename+0x369/0xfd0
>   xfs_vn_rename+0xfa/0x170
>   vfs_rename+0x5fb/0xc30
>   do_renameat2+0x52d/0x6e0
>   __x64_sys_renameat2+0x4b/0x60
>   do_syscall_64+0x3b/0xe0
>   entry_SYSCALL_64_after_hwframe+0x46/0x4e
> 
> A later refactoring patch in the online repair series fixed this by
> accident, which is why I didn't notice this until I started testing only
> the patches that are likely to end up in 6.8.
> 
> Fixes: 1c7ce115e521 ("xfs: reap large AG metadata extents when possible")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/scrub/reap.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/scrub/reap.c b/fs/xfs/scrub/reap.c
> index 9b6c919db522..f99eca799809 100644
> --- a/fs/xfs/scrub/reap.c
> +++ b/fs/xfs/scrub/reap.c
> @@ -251,7 +251,7 @@ xreap_agextent_binval(
>  		max_fsbs = min_t(xfs_agblock_t, agbno_next - bno,
>  				xfs_attr3_rmt_blocks(mp, XFS_XATTR_SIZE_MAX));
>  
> -		for (fsbcount = 1; fsbcount < max_fsbs; fsbcount++) {
> +		for (fsbcount = 1; fsbcount <= max_fsbs; fsbcount++) {
>  			struct xfs_buf	*bp = NULL;
>  			xfs_daddr_t	daddr;
>  			int		error;

Looks good.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com