From: Dave Chinner <dchinner@redhat.com>

Date: Thu, 27 Apr 2023 09:02:11 +1000

Subject: xfs: fix livelock in delayed allocation at ENOSPC

Git-commit: 9419092fb2630c30e4ffeb9ef61007ef0c61827a

Patch-mainline: 6.4-rc1

References: brc#2208553 xfs-issue

On a filesystem with a non-zero stripe unit and a large sequential

write, delayed allocation will set a minimum allocation length of

the stripe unit. If allocation fails because there are no extents

long enough for an aligned minlen allocation, it is supposed to

fall back to unaligned allocation which allows single block extents

to be allocated.

When the allocator code was rewritting in the 6.3 cycle, this

fallback was broken - the old code used args->fsbno as the both the

allocation target and the allocation result, the new code passes the

target as a separate parameter. The conversion didn't handle the

aligned->unaligned fallback path correctly - it reset args->fsbno to

the target fsbno on failure which broke allocation failure detection

in the high level code and so it never fell back to unaligned

allocations.

This resulted in a loop in writeback trying to allocate an aligned

block, getting a false positive success, trying to insert the result

in the BMBT. This did nothing because the extent already was in the

BMBT (merge results in an unchanged extent) and so it returned the

prior extent to the conversion code as the current iomap.

Because the iomap returned didn't cover the offset we tried to map,

xfs_convert_blocks() then retries the allocation, which fails in the

same way and now we have a livelock.

Reported-and-tested-by: Brian Foster <bfoster@redhat.com>

Fixes: 85843327094f ("xfs: factor xfs_bmap_btalloc()")

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

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

---

 fs/xfs/libxfs/xfs_bmap.c |    1 -

 1 file changed, 1 deletion(-)

--- a/fs/xfs/libxfs/xfs_bmap.c

+++ b/fs/xfs/libxfs/xfs_bmap.c

@@ -3505,7 +3505,6 @@ xfs_bmap_btalloc_at_eof(

 	 * original non-aligned state so the caller can proceed on allocation

 	 * failure as if this function was never called.

 	 */

-	args->fsbno = ap->blkno;

 	args->alignment = 1;

 	return 0;

 }