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

[Darrick J. Wong]

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@xxxxxxxxxx>
> ---
>  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
> 
>