Re: [PATCH RFC 06/10] xfs: iomap CoW-based atomic write support

[Darrick J. Wong]

On Tue, Feb 04, 2025 at 12:01:23PM +0000, John Garry wrote:
> In cases of an atomic write occurs for misaligned or discontiguous disk
> blocks, we will use a CoW-based method to issue the atomic write.
> 
> So, for that case, return -EAGAIN to request that the write be issued in
> CoW atomic write mode. The dio write path should detect this, similar to
> how misaligned regalar DIO writes are handled.
> 
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_iomap.c | 68 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 66 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index ae3755ed00e6..2c2867d728e4 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -809,9 +809,12 @@ xfs_direct_write_iomap_begin(
>  	struct xfs_bmbt_irec	imap, cmap;
>  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
>  	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, length);
> +	bool			atomic = flags & IOMAP_ATOMIC;
>  	int			nimaps = 1, error = 0;
>  	bool			shared = false;
> +	bool			found = false;
>  	u16			iomap_flags = 0;
> +	bool			need_alloc;
>  	unsigned int		lockmode;
>  	u64			seq;
>  
> @@ -832,7 +835,7 @@ xfs_direct_write_iomap_begin(
>  	 * COW writes may allocate delalloc space or convert unwritten COW
>  	 * extents, so we need to make sure to take the lock exclusively here.
>  	 */
> -	if (xfs_is_cow_inode(ip))
> +	if (xfs_is_cow_inode(ip) || atomic)
>  		lockmode = XFS_ILOCK_EXCL;
>  	else
>  		lockmode = XFS_ILOCK_SHARED;
> @@ -857,12 +860,73 @@ xfs_direct_write_iomap_begin(
>  	if (error)
>  		goto out_unlock;
>  
> +
> +	if (flags & IOMAP_ATOMIC_COW) {
> +		error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
> +				&lockmode,
> +				(flags & IOMAP_DIRECT) || IS_DAX(inode), true);

Weird nit not relate to this patch: Is there ever a case where
IS_DAX(inode) and (flags & IOMAP_DAX) disagree?  I wonder if this odd
construction could be simplified to:

	(flags & (IOMAP_DIRECT | IOMAP_DAX))

?

> +		if (error)
> +			goto out_unlock;
> +
> +		end_fsb = imap.br_startoff + imap.br_blockcount;
> +		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
> +
> +		if (imap.br_startblock != HOLESTARTBLOCK) {
> +			seq = xfs_iomap_inode_sequence(ip, 0);
> +
> +			error = xfs_bmbt_to_iomap(ip, srcmap, &imap, flags,
> +				iomap_flags | IOMAP_F_ATOMIC_COW, seq);
> +			if (error)
> +				goto out_unlock;
> +		}
> +		seq = xfs_iomap_inode_sequence(ip, 0);
> +		xfs_iunlock(ip, lockmode);
> +		return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags,
> +					iomap_flags | IOMAP_F_ATOMIC_COW, seq);
> +	}

/me wonders if this should be a separate helper so that the main
xfs_direct_write_iomap_begin doesn't get even longer... but otherwise
the logic in here looks sane.

> +
> +	need_alloc = imap_needs_alloc(inode, flags, &imap, nimaps);
> +
> +	if (atomic) {
> +		/* Use CoW-based method if any of the following fail */
> +		error = -EAGAIN;
> +
> +		/*
> +		 * Lazily use CoW-based method for initial alloc of data.
> +		 * Check br_blockcount for FSes which do not support atomic
> +		 * writes > 1x block.
> +		 */
> +		if (need_alloc && imap.br_blockcount > 1)
> +			goto out_unlock;
> +
> +		/* Misaligned start block wrt size */
> +		if (!IS_ALIGNED(imap.br_startblock, imap.br_blockcount))
> +			goto out_unlock;
> +
> +		/* Discontiguous or mixed extents */
> +		if (!imap_spans_range(&imap, offset_fsb, end_fsb))
> +			goto out_unlock;
> +	}

(Same two comments here.)

> +
>  	if (imap_needs_cow(ip, flags, &imap, nimaps)) {
>  		error = -EAGAIN;
>  		if (flags & IOMAP_NOWAIT)
>  			goto out_unlock;
>  
> +		if (atomic) {
> +			/* Detect whether we're already covered in a cow fork */
> +			error  = xfs_find_trim_cow_extent(ip, &imap, &cmap, &shared, &found);
> +			if (error)
> +				goto out_unlock;
> +
> +			if (shared) {
> +				error = -EAGAIN;
> +				goto out_unlock;

What is this checking?  That something else already created a mapping in
the COW fork, so we want to bail out to get rid of it?

--D

> +			}
> +		}
> +
>  		/* may drop and re-acquire the ilock */
> +		shared = false;
>  		error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
>  				&lockmode,
>  				(flags & IOMAP_DIRECT) || IS_DAX(inode), false);
> @@ -874,7 +938,7 @@ xfs_direct_write_iomap_begin(
>  		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
>  	}
>  
> -	if (imap_needs_alloc(inode, flags, &imap, nimaps))
> +	if (need_alloc)
>  		goto allocate_blocks;
>  
>  	/*
> -- 
> 2.31.1
> 
>