Re: [PATCH RFC 10/10] xfs: Allow block allocator to take an alignment hint

John Garry
2025-02-06T08:10:39Z

On 05/02/2025 19:20, Darrick J. Wong wrote:
> On Tue, Feb 04, 2025 at 12:01:27PM +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 40ad22fb808b..7a3910018dee 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_NALIGN)
>> + 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,
>> @@ -3781,7 +3787,6 @@ 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..d68b594c3fa2 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 */
>> +#define XFS_BMAPI_NALIGN (1u << 11)
>> +
>> #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_NALIGN, "NALIGN" }
>
> Tihs isn't really "naturally" aligned, is it? It really means "try to
> align allocations to the extent size hint", which isn't required to be a
> power of two.

Sure, so I would expect that the user will set extsize/cowextsize
according to the size what we want to do atomics for, and we can align
to that. I don't think that it makes a difference that either extsize
isn't mandated to be a power-of-2.

So then I should rename to XFS_BMAPI_EXTSZALIGN or something like that - ok?

Thanks,
John

>
> --D
>
>>
>>
>> static inline int xfs_bmapi_aflag(int w)
>> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
>> index 60c986300faa..198fb5372f10 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_NALIGN;
>>
>> 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
>>
>>