Re: [PATCH 01/11] xfs: reflink should break pnfs leases before sharing blocks

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 01/11] xfs: reflink should break pnfs leases before sharing blocks
Date: Fri, 26 Jan 2018 10:26:28 -0800	[thread overview]
Message-ID: <20180126182628.GW9068@magnolia> (raw)
In-Reply-To: <20180126090652.GA29115@infradead.org>

On Fri, Jan 26, 2018 at 01:06:52AM -0800, Christoph Hellwig wrote:
> On Tue, Jan 23, 2018 at 06:18:03PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Before we share blocks between files, we need to break the pnfs leases
> > on the layout before we start slicing and dicing the block map.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/xfs_reflink.c |   48 +++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 47 insertions(+), 1 deletion(-)
> > 
> > 
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index 47aea2e..f89a725 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -1245,6 +1245,50 @@ xfs_reflink_remap_blocks(
> >  }
> >  
> >  /*
> > + * Grab the exclusive iolock for a data copy from src to dest, making
> > + * sure to abide vfs locking order (lowest pointer value goes first) and
> > + * breaking the pnfs layout leases on dest before proceeding.  The loop
> > + * is needed because we cannot call the blocking break_layout() with the
> > + * src iolock held, and therefore have to back out both locks.
> > + */
> > +static int
> > +xfs_iolock_two_inodes_and_break_layout(
> > +	struct inode		*src,
> > +	struct inode		*dest)
> > +{
> > +	bool			src_first = src < dest;
> > +	bool			src_last = src > dest;
> 
> I find the double predicates here highly confusing.
> 
> Also the code doesn't seem to handle the src == dest case as
> far as I can tell.

I guess they are confusing; when src == dest, src_first and src_last are
both false.

> > +retry:
> > +	if (src_first) {
> > +		inode_lock(src);
> > +		inode_lock_nested(dest, I_MUTEX_NONDIR2);
> > +	} else {
> > +		inode_lock(dest);
> > +	}
> 
> Shouldn't this be replaced by a call to lock_two_nondirectories?
> Even if that holds both locks over the noon-blocking break_layout
> it makes things a lot simpler and only does an additional rountrip
> for the layouts outstanding slow path.
> 
> > +	error = break_layout(dest, false);
> > +	if (error == -EWOULDBLOCK) {
> > +		inode_unlock(dest);
> > +		if (src_first)
> > +			inode_unlock(src);
> 
> unlock_two_nondirectories?
> 
> > +		error = break_layout(dest, true);
> > +		if (error)
> > +			return error;
> > +		goto retry;
> > +	} else if (error) {
> 
> no need for an else after a goto.
> 
> > +		inode_unlock(dest);
> > +		if (src_first)
> > +			inode_unlock(src);
> 
> unlock_two_nondirectories?
> 
> Also seems like this could be simplified to:
> 
> 	if (error) {
> 		unlock_two_nondirectories()
> 		if (error == -EWOULDBLOCK)
> 			goto retry;
> 		return error;
> 	}
> 
> So I guess the whole thing could simply become something like:
> 
> retry:
> 	lock_two_nondirectories(src, dest);
> 	error = break_layout(dest, false);
> 	if (error) {
> 		unlock_two_nondirectories(src, dest);
> 		if (error == -EWOULDBLOCK)
> 			goto retry;
> 		return error;
> 	}
> 
> and could probably just be inlined into the caller..

Yeah, that's simpler... though at this point I'll have to put all this
into a new series having already pushed to for-next.  :/

(Sorry, kinda overburdened with this week)

--D

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