Re: [PATCH RFC 09/10] xfs: Update atomic write max size

Darrick J. Wong
2025-02-05T19:41:16Z

On Tue, Feb 04, 2025 at 12:01:26PM +0000, John Garry wrote:
> Now that CoW-based atomic writes are supported, update the max size of an
> atomic write.
>
> For simplicity, limit at the max of what the mounted bdev can support in
> terms of atomic write limits. Maybe in future we will have a better way
> to advertise this optimised limit.
>
> In addition, the max atomic write size needs to be aligned to the agsize.
> Currently when attempting to use HW offload, we just check that the
> mapping startblock is aligned. However, that is just the startblock within
> the AG, and the AG may not be properly aligned to the underlying block
> device atomic write limits.
>
> As such, limit atomic writes to the greatest power-of-2 which fits in an
> AG, so that aligning to the startblock will be mean that we are also
> aligned to the disk block.

I don't understand this sentence -- what are we "aligning to the
startblock"? I think you're saying that you want to limit the size of
untorn writes to the greatest power-of-two factor of the agsize so that
allocations for an untorn write will always be aligned compatibly with
the alignment requirements of the storage for an untorn write?

> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
> fs/xfs/xfs_iops.c | 7 ++++++-
> fs/xfs/xfs_mount.c | 28 ++++++++++++++++++++++++++++
> fs/xfs/xfs_mount.h | 1 +
> 3 files changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index ea79fb246e33..95681d6c2bcd 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -606,12 +606,17 @@ xfs_get_atomic_write_attr(
> unsigned int *unit_min,
> unsigned int *unit_max)
> {
> + struct xfs_buftarg *target = xfs_inode_buftarg(ip);
> + struct xfs_mount *mp = ip->i_mount;
> +
> if (!xfs_inode_can_atomicwrite(ip)) {
> *unit_min = *unit_max = 0;
> return;
> }
>
> - *unit_min = *unit_max = ip->i_mount->m_sb.sb_blocksize;
> + *unit_min = ip->i_mount->m_sb.sb_blocksize;
> + *unit_max = min_t(unsigned int, XFS_FSB_TO_B(mp, mp->awu_max),
> + target->bt_bdev_awu_max);
> }
>
> static void
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 477c5262cf91..4e60347f6b7e 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -651,6 +651,32 @@ xfs_agbtree_compute_maxlevels(
> levels = max(levels, mp->m_rmap_maxlevels);
> mp->m_agbtree_maxlevels = max(levels, mp->m_refc_maxlevels);
> }
> +static inline void
> +xfs_mp_compute_awu_max(

xfs_compute_awu_max() ?

> + struct xfs_mount *mp)
> +{
> + xfs_agblock_t agsize = mp->m_sb.sb_agblocks;
> + xfs_agblock_t awu_max;
> +
> + if (!xfs_has_reflink(mp)) {
> + mp->awu_max = 1;
> + return;
> + }
> +
> + /*
> + * Find highest power-of-2 evenly divisible into agsize and which
> + * also fits into an unsigned int field.
> + */
> + awu_max = 1;
> + while (1) {
> + if (agsize % (awu_max * 2))
> + break;
> + if (XFS_FSB_TO_B(mp, awu_max * 2) > UINT_MAX)
> + break;
> + awu_max *= 2;
> + }
> + mp->awu_max = awu_max;

I think you need two awu_maxes here -- one for the data device, and
another for the realtime device. The rt computation is probably more
complex since I think it's the greatest power of two that fits in the rt
extent size if it isn't a power of two; or the greatest power of two
that fits in the rtgroup if rtgroups are enabled; or probably just no
limit otherwise.

--D

> +}
>
> /* Compute maximum possible height for realtime btree types for this fs. */
> static inline void
> @@ -736,6 +762,8 @@ xfs_mountfs(
> xfs_agbtree_compute_maxlevels(mp);
> xfs_rtbtree_compute_maxlevels(mp);
>
> + xfs_mp_compute_awu_max(mp);
> +
> /*
> * Check if sb_agblocks is aligned at stripe boundary. If sb_agblocks
> * is NOT aligned turn off m_dalign since allocator alignment is within
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index fbed172d6770..34286c87ac4a 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -198,6 +198,7 @@ typedef struct xfs_mount {
> bool m_fail_unmount;
> bool m_finobt_nores; /* no per-AG finobt resv. */
> bool m_update_sb; /* sb needs update in mount */
> + xfs_extlen_t awu_max; /* max atomic write */
>
> /*
> * Bitsets of per-fs metadata that have been checked and/or are sick.
> --
> 2.31.1
>
>