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

Darrick J. Wong
2025-02-24T20:37:38Z

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
>
>