Re: [PATCH 09/10] xfs: set IOMAP_F_SHARED for all COW fork allocations

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Aug 27, 2024 at 07:09:56AM +0200, Christoph Hellwig wrote:
> Change to always set xfs_buffered_write_iomap_begin for COW fork
> allocations even if they don't overlap existing data fork extents,
> which will allow the iomap_end callback to detect if it has to punch
> stale delalloc blocks from the COW fork instead of the data fork.  It
> also means we sample the sequence counter for both the data and the COW
> fork when writing to the COW fork, which ensures we properly revalidate
> when only COW fork changes happens.
> 
> This is essentially a revert of commit 72a048c1056a ("xfs: only set
> IOMAP_F_SHARED when providing a srcmap to a write"). This is fine because
> the problem that the commit fixed has now been dealt with in iomap by
> only looking at the actual srcmap and not the fallback to the write
> iomap.
> 
> Note that the direct I/O path was never changed and has always set
> IOMAP_F_SHARED for all COW fork allocations.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

That was fun to compare your revert vs. a patch from 3 years ago. $)

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_iomap.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index e0dc6393686c01..22e9613a995f12 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1186,20 +1186,19 @@ xfs_buffered_write_iomap_begin(
>  	return 0;
>  
>  found_cow:
> -	seq = xfs_iomap_inode_sequence(ip, 0);
>  	if (imap.br_startoff <= offset_fsb) {
> -		error = xfs_bmbt_to_iomap(ip, srcmap, &imap, flags, 0, seq);
> +		error = xfs_bmbt_to_iomap(ip, srcmap, &imap, flags, 0,
> +				xfs_iomap_inode_sequence(ip, 0));
>  		if (error)
>  			goto out_unlock;
> -		seq = xfs_iomap_inode_sequence(ip, IOMAP_F_SHARED);
> -		xfs_iunlock(ip, lockmode);
> -		return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags,
> -					 IOMAP_F_SHARED, seq);
> +	} else {
> +		xfs_trim_extent(&cmap, offset_fsb, imap.br_startoff - offset_fsb);
>  	}
>  
> -	xfs_trim_extent(&cmap, offset_fsb, imap.br_startoff - offset_fsb);
> +	iomap_flags = IOMAP_F_SHARED;
> +	seq = xfs_iomap_inode_sequence(ip, iomap_flags);
>  	xfs_iunlock(ip, lockmode);
> -	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, 0, seq);
> +	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, iomap_flags, seq);
>  
>  out_unlock:
>  	xfs_iunlock(ip, lockmode);
> -- 
> 2.43.0
> 
>