linux-kernel - Re: [PATCH v6 5/7] xfs: Support atomic write for statx

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Message-ID: <20240930163716.GO21853@frogsfrogsfrogs>
Date: Mon, 30 Sep 2024 09:37:16 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: axboe@...nel.dk, brauner@...nel.org, viro@...iv.linux.org.uk,
	jack@...e.cz, dchinner@...hat.com, hch@....de, cem@...nel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hare@...e.de, martin.petersen@...cle.com,
	catherine.hoang@...cle.com, mcgrof@...nel.org,
	ritesh.list@...il.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH v6 5/7] xfs: Support atomic write for statx

On Mon, Sep 30, 2024 at 12:54:36PM +0000, John Garry wrote:
> Support providing info on atomic write unit min and max for an inode.
> 
> For simplicity, currently we limit the min at the FS block size. As for
> max, we limit also at FS block size, as there is no current method to
> guarantee extent alignment or granularity for regular files.
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  fs/xfs/xfs_inode.h | 17 +++++++++++++++++
>  fs/xfs/xfs_iops.c  | 24 ++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 1c62ee294a5a..1ea73402d592 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -332,6 +332,23 @@ static inline bool xfs_inode_has_atomicwrites(struct xfs_inode *ip)
>  	return ip->i_diflags2 & XFS_DIFLAG2_ATOMICWRITES;
>  }
>  
> +static inline bool
> +xfs_inode_can_atomicwrite(
> +	struct xfs_inode	*ip)
> +{
> +	struct xfs_mount	*mp = ip->i_mount;
> +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> +
> +	if (!xfs_inode_has_atomicwrites(ip))
> +		return false;
> +	if (mp->m_sb.sb_blocksize < target->bt_bdev_awu_min)
> +		return false;
> +	if (mp->m_sb.sb_blocksize > target->bt_bdev_awu_max)
> +		return false;
> +
> +	return true;
> +}
> +
>  /*
>   * In-core inode flags.
>   */
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index ee79cf161312..915d057db9bb 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -570,6 +570,23 @@ xfs_stat_blksize(
>  	return max_t(uint32_t, PAGE_SIZE, mp->m_sb.sb_blocksize);
>  }
>  
> +static void
> +xfs_get_atomic_write_attr(
> +	struct xfs_inode	*ip,
> +	unsigned int		*unit_min,
> +	unsigned int		*unit_max)
> +{
> +	struct xfs_mount	*mp = ip->i_mount;
> +	struct xfs_sb		*sbp = &mp->m_sb;
> +
> +	if (!xfs_inode_can_atomicwrite(ip)) {
> +		*unit_min = *unit_max = 0;
> +		return;
> +	}
> +
> +	*unit_min = *unit_max = sbp->sb_blocksize;

Ok, so we're only supporting untorn writes if they're exactly the fs
blocksize, and 1 fsblock is between awu_min/max.  That simplifies a lot
of things. :)

Not supporting sub-fsblock atomic writes means that we'll never hit the
directio COW fallback code, which uses the pagecache.

Not supporting multi-fsblock atomic writes means that you don't have to
figure out how to ensure that we always do cow on forcealign
granularity.  Though as I pointed out elsewhere in this thread, that's a
forcealign problem.

Yay! ;)

> +}
> +
>  STATIC int
>  xfs_vn_getattr(
>  	struct mnt_idmap	*idmap,
> @@ -643,6 +660,13 @@ xfs_vn_getattr(
>  			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
>  			stat->dio_offset_align = bdev_logical_block_size(bdev);
>  		}
> +		if (request_mask & STATX_WRITE_ATOMIC) {
> +			unsigned int unit_min, unit_max;
> +
> +			xfs_get_atomic_write_attr(ip, &unit_min, &unit_max);
> +			generic_fill_statx_atomic_writes(stat,
> +				unit_min, unit_max);

Consistent indenting and wrapping, please:

			xfs_get_atomic_write_attr(ip, &unit_min,
					&unit_max);
			generic_fill_statx_atomic_writes(stat,
					unit_min, unit_max);


With that fixed,
Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D


> +		}
>  		fallthrough;
>  	default:
>  		stat->blksize = xfs_stat_blksize(ip);
> -- 
> 2.31.1
> 
>