Re: [PATCH] xfs: fix livelock in delayed allocation at ENOSPC

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Apr 22, 2023 at 08:24:40AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> 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-by: Brian Foster <bfoster@redhat.com>
> Fixes: 85843327094f ("xfs: factor xfs_bmap_btalloc()")
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Will give this one a spin through the test system over the weekend.

In the meantime, can one of you come up with a reproducer?  From the
description, it doesn't sound like that should be too hard -- mount with
no stripe unit set, fragment the free space, mount with a stripe unit
set, then run the fs out of space?

--D

> ---
>  fs/xfs/libxfs/xfs_bmap.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 1a4e446194dd..b512de0540d5 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3540,7 +3540,6 @@ 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;
>  }
> -- 
> 2.39.2
>