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

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Feb 06, 2025 at 10:27:45AM +0000, John Garry wrote:
> On 05/02/2025 19:47, Darrick J. Wong wrote:
> > 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,
> 
> nor do I ...
> 
> > but it's roughly
> > tr_logres divided by the average size of a log intent item.
> 
> So you have a ballpark figure on the average size of a log intent item, or
> an idea on how to get it?

You could add up the size of struct
xfs_{bui,rmap,refcount,efi}_log_format structures and add 20%, that will
give you a ballpark figure of the worst case per-block requirements.

My guess is that 64 blocks is ok provided resblks is big enough.  But I
guess we could estimate it (very conservatively) dynamically too.

(also note tr_itruncate declares more logres)

> > This means we need to restrict the size of an untorn write to a
> > double-digit number of fsblocks for safety.
> 
> Sure, but won't we also still be liable to suffer the same issue which was
> fixed in commit d6f215f359637?

Yeah, come to think of it, you need to reserve the worst case space
reservation, i.e. each of the blocks between offset_fsb and end_fsb
becomes a separate btree update.

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

--D

> > 
> > The logic in here looks reasonable though.
> > 
> 
> Thanks,
> John
> 
> > --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
> > > 
> > > 
> 
>