Re: [PATCH v2 11/11] xfs: Allow block allocator to take an alignment hint

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

On Thu, Feb 13, 2025 at 01:56:19PM +0000, John Garry wrote:
> When issuing an atomic write by the CoW method, give the block allocator a
> hint to naturally align the data blocks.
> 
> This means that we have a better chance to issuing the atomic write via
> HW offload next time.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 7 ++++++-
>  fs/xfs/libxfs/xfs_bmap.h | 6 +++++-
>  fs/xfs/xfs_reflink.c     | 8 ++++++--
>  3 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 0ef19f1469ec..9bfdfb7cdcae 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3454,6 +3454,12 @@ xfs_bmap_compute_alignments(
>  		align = xfs_get_cowextsz_hint(ap->ip);
>  	else if (ap->datatype & XFS_ALLOC_USERDATA)
>  		align = xfs_get_extsz_hint(ap->ip);
> +
> +	if (align > 1 && ap->flags & XFS_BMAPI_EXTSZALIGN)
> +		args->alignment = align;
> +	else
> +		args->alignment = 1;
> +
>  	if (align) {
>  		if (xfs_bmap_extsize_align(mp, &ap->got, &ap->prev, align, 0,
>  					ap->eof, 0, ap->conv, &ap->offset,
> @@ -3782,7 +3788,6 @@ xfs_bmap_btalloc(
>  		.wasdel		= ap->wasdel,
>  		.resv		= XFS_AG_RESV_NONE,
>  		.datatype	= ap->datatype,
> -		.alignment	= 1,
>  		.minalignslop	= 0,
>  	};
>  	xfs_fileoff_t		orig_offset;
> diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> index 4b721d935994..7a31697331dc 100644
> --- a/fs/xfs/libxfs/xfs_bmap.h
> +++ b/fs/xfs/libxfs/xfs_bmap.h
> @@ -87,6 +87,9 @@ struct xfs_bmalloca {
>  /* Do not update the rmap btree.  Used for reconstructing bmbt from rmapbt. */
>  #define XFS_BMAPI_NORMAP	(1u << 10)
>  
> +/* Try to naturally align allocations to extsz hint */
> +#define XFS_BMAPI_EXTSZALIGN	(1u << 11)

IMO "naturally" makes things confusing here -- are we aligning to the
extent size hint, or are we aligning to the length requested?  Or
whatever it is that "naturally" means.

(FWIW you and I have bumped over this repeatedly, so maybe this is
simple one of those cognitive friction things where block storage always
deals with powers of two and "naturally" means a lot, vs. filesystems
where we don't usually enforce alignment anywhere.)

I suggest "Try to align allocations to the extent size hint" for the
comment, and with that:
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> +
>  #define XFS_BMAPI_FLAGS \
>  	{ XFS_BMAPI_ENTIRE,	"ENTIRE" }, \
>  	{ XFS_BMAPI_METADATA,	"METADATA" }, \
> @@ -98,7 +101,8 @@ struct xfs_bmalloca {
>  	{ XFS_BMAPI_REMAP,	"REMAP" }, \
>  	{ XFS_BMAPI_COWFORK,	"COWFORK" }, \
>  	{ XFS_BMAPI_NODISCARD,	"NODISCARD" }, \
> -	{ XFS_BMAPI_NORMAP,	"NORMAP" }
> +	{ XFS_BMAPI_NORMAP,	"NORMAP" },\
> +	{ XFS_BMAPI_EXTSZALIGN,	"EXTSZALIGN" }
>  
>  
>  static inline int xfs_bmapi_aflag(int w)
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index d097d33dc000..033dff86ecf0 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -445,6 +445,11 @@ xfs_reflink_fill_cow_hole(
>  	int			nimaps;
>  	int			error;
>  	bool			found;
> +	uint32_t		bmapi_flags = XFS_BMAPI_COWFORK |
> +					      XFS_BMAPI_PREALLOC;
> +
> +	if (atomic)
> +		bmapi_flags |= XFS_BMAPI_EXTSZALIGN;
>  
>  	resaligned = xfs_aligned_fsb_count(imap->br_startoff,
>  		imap->br_blockcount, xfs_get_cowextsz_hint(ip));
> @@ -478,8 +483,7 @@ xfs_reflink_fill_cow_hole(
>  	/* Allocate the entire reservation as unwritten blocks. */
>  	nimaps = 1;
>  	error = xfs_bmapi_write(tp, ip, imap->br_startoff, imap->br_blockcount,
> -			XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC, 0, cmap,
> -			&nimaps);
> +			bmapi_flags, 0, cmap, &nimaps);
>  	if (error)
>  		goto out_trans_cancel;
>  
> -- 
> 2.31.1
> 
>