Re: [PATCH v2 09/11] xfs: Commit CoW-based atomic writes atomically

Darrick J. Wong
2025-02-24T20:20:35Z

On Thu, Feb 13, 2025 at 01:56:17PM +0000, John Garry wrote:
> When completing a CoW-based write, each extent range mapping update is
> covered by a separate transaction.
>
> For a CoW-based atomic write, all mappings must be changed at once, so
> change to use a single transaction.
>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
> fs/xfs/xfs_file.c | 5 ++++-
> fs/xfs/xfs_reflink.c | 45 ++++++++++++++++++++++++++++++++++++++++++++
> fs/xfs/xfs_reflink.h | 3 +++
> 3 files changed, 52 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 9762fa503a41..243640fe4874 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -527,7 +527,10 @@ xfs_dio_write_end_io(
> nofs_flag = memalloc_nofs_save();
>
> if (flags & IOMAP_DIO_COW) {
> - error = xfs_reflink_end_cow(ip, offset, size);
> + if (iocb->ki_flags & IOCB_ATOMIC)
> + error = xfs_reflink_end_atomic_cow(ip, offset, size);
> + else
> + error = xfs_reflink_end_cow(ip, offset, size);
> if (error)
> goto out;
> }
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index 3dab3ba900a3..d097d33dc000 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -986,6 +986,51 @@ xfs_reflink_end_cow(
> trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
> return error;
> }
> +int
> +xfs_reflink_end_atomic_cow(
> + struct xfs_inode *ip,
> + xfs_off_t offset,
> + xfs_off_t count)
> +{
> + xfs_fileoff_t offset_fsb;
> + xfs_fileoff_t end_fsb;
> + int error = 0;
> + struct xfs_mount *mp = ip->i_mount;
> + struct xfs_trans *tp;
> + unsigned int resblks;
> +
> + trace_xfs_reflink_end_cow(ip, offset, count);
> +
> + offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
> + end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);

Use @mp here instead of walking the pointer.

> +
> + resblks = (end_fsb - offset_fsb) *
> + XFS_NEXTENTADD_SPACE_RES(mp, 1, XFS_DATA_FORK);

How did you arrive at this computation? The "b" parameter to
XFS_NEXTENTADD_SPACE_RES is usually the worst case number of mappings
that you're going to change on this file. I think that quantity is
(end_fsb - offset_fsb)?

> +
> + error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
> + XFS_TRANS_RESERVE, &tp);
> + if (error)
> + return error;
> +
> + xfs_ilock(ip, XFS_ILOCK_EXCL);
> + xfs_trans_ijoin(tp, ip, 0);
> +
> + while (end_fsb > offset_fsb && !error)
> + error = xfs_reflink_end_cow_extent_locked(tp, ip, &offset_fsb,
> + end_fsb);

Overly long line, and the continuation line only needs to be indented
two more tabs.

> +
> + if (error) {
> + trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
> + goto out_cancel;
> + }
> + error = xfs_trans_commit(tp);
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> + return 0;

Why is it ok to drop @error here? Shouldn't a transaction commit error
should be reported to the writer thread?

--D

> +out_cancel:
> + xfs_trans_cancel(tp);
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> + return error;
> +}
>
> /*
> * Free all CoW staging blocks that are still referenced by the ondisk refcount
> diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
> index 754d2bb692d3..ca945d98e823 100644
> --- a/fs/xfs/xfs_reflink.h
> +++ b/fs/xfs/xfs_reflink.h
> @@ -43,6 +43,9 @@ extern int xfs_reflink_cancel_cow_range(struct xfs_inode *ip, xfs_off_t offset,
> xfs_off_t count, bool cancel_real);
> extern int xfs_reflink_end_cow(struct xfs_inode *ip, xfs_off_t offset,
> xfs_off_t count);
> + int
> +xfs_reflink_end_atomic_cow(struct xfs_inode *ip, xfs_off_t offset,
> + xfs_off_t count);
> extern int xfs_reflink_recover_cow(struct xfs_mount *mp);
> extern loff_t xfs_reflink_remap_range(struct file *file_in, loff_t pos_in,
> struct file *file_out, loff_t pos_out, loff_t len,
> --
> 2.31.1
>
>