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

Darrick J. Wong
2025-02-05T20:05:17Z

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@oracle.com>
> ---
> 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
>
>