Re: [PATCH 1/2] xfs: fix delalloc write failures in software-provided atomic writes

[John Garry <john.g.garry@oracle.com>]

On 03/11/2025 17:40, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> With the 20 Oct 2025 release of fstests, generic/521 fails for me on
> regular (aka non-block-atomic-writes) storage:
> 
> QA output created by 521
> dowrite: write: Input/output error
> LOG DUMP (8553 total operations):
> 1(  1 mod 256): SKIPPED (no operation)
> 2(  2 mod 256): WRITE    0x7e000 thru 0x8dfff	(0x10000 bytes) HOLE
> 3(  3 mod 256): READ     0x69000 thru 0x79fff	(0x11000 bytes)
> 4(  4 mod 256): FALLOC   0x53c38 thru 0x5e853	(0xac1b bytes) INTERIOR
> 5(  5 mod 256): COPY 0x55000 thru 0x59fff	(0x5000 bytes) to 0x25000 thru 0x29fff
> 6(  6 mod 256): WRITE    0x74000 thru 0x88fff	(0x15000 bytes)
> 7(  7 mod 256): ZERO     0xedb1 thru 0x11693	(0x28e3 bytes)
> 
> with a warning in dmesg from iomap about XFS trying to give it a
> delalloc mapping for a directio write.  Fix the software atomic write
> iomap_begin code to convert the reservation into a written mapping.
> This doesn't fix the data corruption problems reported by generic/760,
> but it's a start.
> 
> Cc: <stable@vger.kernel.org> # v6.16
> Fixes: bd1d2c21d5d249 ("xfs: add xfs_atomic_write_cow_iomap_begin()")
> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>

FWIW:

Reviewed-by: John Garry <john.g.garry@oracle.com>

> ---
>   fs/xfs/xfs_iomap.c |   21 +++++++++++++++++++--
>   1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index d3f6e3e42a1191..e1da06b157cf94 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1130,7 +1130,7 @@ xfs_atomic_write_cow_iomap_begin(
>   		return -EAGAIN;
>   
>   	trace_xfs_iomap_atomic_write_cow(ip, offset, length);
> -
> +retry:
>   	xfs_ilock(ip, XFS_ILOCK_EXCL);
>   
>   	if (!ip->i_cowfp) {
> @@ -1141,6 +1141,8 @@ xfs_atomic_write_cow_iomap_begin(
>   	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
>   		cmap.br_startoff = end_fsb;
>   	if (cmap.br_startoff <= offset_fsb) {
> +		if (isnullstartblock(cmap.br_startblock))

This following comment is unrelated to this patch and is only relevant 
to pre-existing code:

isnullstartblock() seems to be a check specific to delayed allocation, 
so I don't why "null" is used in the name, and not "delalloc" or 
something else more specific.

I guess that there is some history here (behind the naming).

> +			goto convert;
>   		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
>   		goto found;
>   	}
> @@ -1169,8 +1171,10 @@ xfs_atomic_write_cow_iomap_begin(
>   	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
>   		cmap.br_startoff = end_fsb;
>   	if (cmap.br_startoff <= offset_fsb) {
> -		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
>   		xfs_trans_cancel(tp);
> +		if (isnullstartblock(cmap.br_startblock))
> +			goto convert;
> +		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
>   		goto found;
>   	}
>   
> @@ -1210,6 +1214,19 @@ xfs_atomic_write_cow_iomap_begin(
>   	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>   	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, IOMAP_F_SHARED, seq);
>   
> +convert:

minor comment:

could convert_delay be a better name, like used in 
xfs_buffered_write_iomap_begin()?

> +	xfs_iunlock(ip, XFS_ILOCK_EXCL);
> +	error = xfs_bmapi_convert_delalloc(ip, XFS_COW_FORK, offset, iomap,
> +			NULL);
> +	if (error)
> +		return error;
> +
> +	/*
> +	 * Try the lookup again, because the delalloc conversion might have
> +	 * turned the COW mapping into unwritten, but we need it to be in
> +	 * written state.
> +	 */
> +	goto retry;
>   out_unlock:
>   	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>   	return error;