Re: [PATCH 1/6] xfs: move the dio write relocking out of xfs_ilock_for_iomap

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

On Wed, Jun 19, 2024 at 01:53:51PM +0200, Christoph Hellwig wrote:
> About half of xfs_ilock_for_iomap deals with a special case for direct
> I/O writes to COW files that need to take the ilock exclusively.  Move
> this code into the one callers that cares and simplify
> xfs_ilock_for_iomap.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

It took me a few minutes to sort this out, but yeah, directio writes are
the only place where we do this shared->cow->excl promotion dance.

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

--D

> ---
>  fs/xfs/xfs_iomap.c | 71 ++++++++++++++++++++++------------------------
>  1 file changed, 34 insertions(+), 37 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 3783426739258c..b0085e5972393a 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -717,53 +717,30 @@ imap_needs_cow(
>  	return true;
>  }
>  
> +/*
> + * Extents not yet cached requires exclusive access, don't block for
> + * IOMAP_NOWAIT.
> + *
> + * This is basically an opencoded xfs_ilock_data_map_shared() call, but with
> + * support for IOMAP_NOWAIT.
> + */
>  static int
>  xfs_ilock_for_iomap(
>  	struct xfs_inode	*ip,
>  	unsigned		flags,
>  	unsigned		*lockmode)
>  {
> -	unsigned int		mode = *lockmode;
> -	bool			is_write = flags & (IOMAP_WRITE | IOMAP_ZERO);
> -
> -	/*
> -	 * 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) && is_write)
> -		mode = XFS_ILOCK_EXCL;
> -
> -	/*
> -	 * Extents not yet cached requires exclusive access, don't block.  This
> -	 * is an opencoded xfs_ilock_data_map_shared() call but with
> -	 * non-blocking behaviour.
> -	 */
> -	if (xfs_need_iread_extents(&ip->i_df)) {
> -		if (flags & IOMAP_NOWAIT)
> -			return -EAGAIN;
> -		mode = XFS_ILOCK_EXCL;
> -	}
> -
> -relock:
>  	if (flags & IOMAP_NOWAIT) {
> -		if (!xfs_ilock_nowait(ip, mode))
> +		if (xfs_need_iread_extents(&ip->i_df))
> +			return -EAGAIN;
> +		if (!xfs_ilock_nowait(ip, *lockmode))
>  			return -EAGAIN;
>  	} else {
> -		xfs_ilock(ip, mode);
> +		if (xfs_need_iread_extents(&ip->i_df))
> +			*lockmode = XFS_ILOCK_EXCL;
> +		xfs_ilock(ip, *lockmode);
>  	}
>  
> -	/*
> -	 * The reflink iflag could have changed since the earlier unlocked
> -	 * check, so if we got ILOCK_SHARED for a write and but we're now a
> -	 * reflink inode we have to switch to ILOCK_EXCL and relock.
> -	 */
> -	if (mode == XFS_ILOCK_SHARED && is_write && xfs_is_cow_inode(ip)) {
> -		xfs_iunlock(ip, mode);
> -		mode = XFS_ILOCK_EXCL;
> -		goto relock;
> -	}
> -
> -	*lockmode = mode;
>  	return 0;
>  }
>  
> @@ -801,7 +778,7 @@ xfs_direct_write_iomap_begin(
>  	int			nimaps = 1, error = 0;
>  	bool			shared = false;
>  	u16			iomap_flags = 0;
> -	unsigned int		lockmode = XFS_ILOCK_SHARED;
> +	unsigned int		lockmode;
>  	u64			seq;
>  
>  	ASSERT(flags & (IOMAP_WRITE | IOMAP_ZERO));
> @@ -817,10 +794,30 @@ xfs_direct_write_iomap_begin(
>  	if (offset + length > i_size_read(inode))
>  		iomap_flags |= IOMAP_F_DIRTY;
>  
> +	/*
> +	 * 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))
> +		lockmode = XFS_ILOCK_EXCL;
> +	else
> +		lockmode = XFS_ILOCK_SHARED;
> +
> +relock:
>  	error = xfs_ilock_for_iomap(ip, flags, &lockmode);
>  	if (error)
>  		return error;
>  
> +	/*
> +	 * The reflink iflag could have changed since the earlier unlocked
> +	 * check, check if it again and relock if needed.
> +	 */
> +	if (xfs_is_cow_inode(ip) && lockmode == XFS_ILOCK_SHARED) {
> +		xfs_iunlock(ip, lockmode);
> +		lockmode = XFS_ILOCK_EXCL;
> +		goto relock;
> +	}
> +
>  	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
>  			       &nimaps, 0);
>  	if (error)
> -- 
> 2.43.0
> 
>