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