Re: [PATCH RFC 08/10] xfs: Commit CoW-based atomic writes atomically

Darrick J. Wong
2025-02-05T19:47:40Z

On Tue, Feb 04, 2025 at 12:01:25PM +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 | 48 ++++++++++++++++++++++++++++++++++++++++++++
> fs/xfs/xfs_reflink.h | 3 +++
> 3 files changed, 55 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 12af5cdc3094..170d7891f90d 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 dbce333b60eb..60c986300faa 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -990,6 +990,54 @@ 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;
> + bool commit = false;
> +
> + 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);
> +
> + resblks = XFS_NEXTENTADD_SPACE_RES(ip->i_mount,
> + (unsigned int)(end_fsb - offset_fsb),
> + XFS_DATA_FORK);
> +
> + error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,

xfs gained reflink support for realtime volumes in 6.14-rc1, so you now
have to calculate for that in here too.

> + 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(ip, &offset_fsb,
> + end_fsb, tp, &commit);

Hmm. Attaching intent items to a transaction consumes space in that
transaction, so we probably ought to limit the amount that we try to do
here. Do you know what that limit is? I don't, but it's roughly
tr_logres divided by the average size of a log intent item.

This means we need to restrict the size of an untorn write to a
double-digit number of fsblocks for safety.

The logic in here looks reasonable though.

--D

> +
> + if (error || !commit)
> + goto out_cancel;
> +
> + if (error)
> + trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
> + error = xfs_trans_commit(tp);
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> + return error;
> +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 ef5c8b2398d8..2c3b096c1386 100644
> --- a/fs/xfs/xfs_reflink.h
> +++ b/fs/xfs/xfs_reflink.h
> @@ -45,6 +45,9 @@ 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
>
>