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

Darrick J. Wong
2024-06-20T18:40:34Z

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
>
>