Re: [PATCH 06/13] xfs: align args->minlen for forced allocation alignment

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

On Fri, Jun 21, 2024 at 10:05:33AM +0000, John Garry wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> If args->minlen is not aligned to the constraints of forced
> alignment, we may do minlen allocations that are not aligned when we
> approach ENOSPC. Avoid this by always aligning args->minlen
> appropriately. If alignment of minlen results in a value smaller
> than the alignment constraint, fail the allocation immediately.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: John Garry <john.g.garry@oracle.com>

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

--D

> ---
>  fs/xfs/libxfs/xfs_bmap.c | 45 +++++++++++++++++++++++++++-------------
>  1 file changed, 31 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 9131ba8113a6..c9cf138e13c4 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3278,33 +3278,48 @@ xfs_bmap_longest_free_extent(
>  	return 0;
>  }
>  
> -static xfs_extlen_t
> +static int
>  xfs_bmap_select_minlen(
>  	struct xfs_bmalloca	*ap,
>  	struct xfs_alloc_arg	*args,
>  	xfs_extlen_t		blen)
>  {
> -
>  	/* Adjust best length for extent start alignment. */
>  	if (blen > args->alignment)
>  		blen -= args->alignment;
>  
>  	/*
>  	 * Since we used XFS_ALLOC_FLAG_TRYLOCK in _longest_free_extent(), it is
> -	 * possible that there is enough contiguous free space for this request.
> +	 * possible that there is enough contiguous free space for this request
> +	 * even if best length is less that the minimum length we need.
> +	 *
> +	 * If the best length won't satisfy the maximum length we requested,
> +	 * then use it as the minimum length so we get as large an allocation
> +	 * as possible.
>  	 */
>  	if (blen < ap->minlen)
> -		return ap->minlen;
> +		blen = ap->minlen;
> +	else if (blen > args->maxlen)
> +		blen = args->maxlen;
>  
>  	/*
> -	 * If the best seen length is less than the request length,
> -	 * use the best as the minimum, otherwise we've got the maxlen we
> -	 * were asked for.
> +	 * If we have alignment constraints, round the minlen down to match the
> +	 * constraint so that alignment will be attempted. This may reduce the
> +	 * allocation to smaller than was requested, so clamp the minimum to
> +	 * ap->minlen to allow unaligned allocation to succeed. If we are forced
> +	 * to align the allocation, return ENOSPC at this point because we don't
> +	 * have enough contiguous free space to guarantee aligned allocation.
>  	 */
> -	if (blen < args->maxlen)
> -		return blen;
> -	return args->maxlen;
> -
> +	if (args->alignment > 1) {
> +		blen = rounddown(blen, args->alignment);
> +		if (blen < ap->minlen) {
> +			if (args->datatype & XFS_ALLOC_FORCEALIGN)
> +				return -ENOSPC;
> +			blen = ap->minlen;
> +		}
> +	}
> +	args->minlen = blen;
> +	return 0;
>  }
>  
>  static int
> @@ -3340,8 +3355,7 @@ xfs_bmap_btalloc_select_lengths(
>  	if (pag)
>  		xfs_perag_rele(pag);
>  
> -	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> -	return error;
> +	return xfs_bmap_select_minlen(ap, args, blen);
>  }
>  
>  /* Update all inode and quota accounting for the allocation we just did. */
> @@ -3661,7 +3675,10 @@ xfs_bmap_btalloc_filestreams(
>  		goto out_low_space;
>  	}
>  
> -	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> +	error = xfs_bmap_select_minlen(ap, args, blen);
> +	if (error)
> +		goto out_low_space;
> +
>  	if (ap->aeof && ap->offset)
>  		error = xfs_bmap_btalloc_at_eof(ap, args);
>  
> -- 
> 2.31.1
> 
>