Re: [PATCH v3 2/2] ext4: handle layout changes to pinned DAX mappings

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Wed, Jul 04, 2018 at 08:59:52PM -0700, Darrick J. Wong wrote:
> On Thu, Jul 05, 2018 at 09:54:14AM +1000, Dave Chinner wrote:
> > On Wed, Jul 04, 2018 at 02:27:23PM +0200, Jan Kara wrote:
> > > On Wed 04-07-18 10:49:23, Dave Chinner wrote:
> > > > On Mon, Jul 02, 2018 at 11:29:12AM -0600, Ross Zwisler wrote:
> > > > > Follow the lead of xfs_break_dax_layouts() and add synchronization between
> > > > > operations in ext4 which remove blocks from an inode (hole punch, truncate
> > > > > down, etc.) and pages which are pinned due to DAX DMA operations.
> > > > > 
> > > > > Signed-off-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> > > > > Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
> > > > > Reviewed-by: Lukas Czerner <lczerner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > > > > ---
> > > > > 
> > > > > Changes since v2:
> > > > >  * Added a comment to ext4_insert_range() explaining why we don't call
> > > > >    ext4_break_layouts(). (Jan)
> > > > 
> > > > Which I think is wrong and will cause data corruption.
> > > > 
> > > > > @@ -5651,6 +5663,11 @@ int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)
> > > > >  			LLONG_MAX);
> > > > >  	if (ret)
> > > > >  		goto out_mmap;
> > > > > +	/*
> > > > > +	 * We don't need to call ext4_break_layouts() because we aren't
> > > > > +	 * removing any blocks from the inode.  We are just changing their
> > > > > +	 * offset by inserting a hole.
> > > > > +	 */
> 
> Does calling ext4_break_layouts from insert range not work?
> 
> It's my understanding that file leases work are a mechanism for the
> filesystem to delegate some of its authority over physical space
> mappings to "client" software.  AFAICT it's used for mmap'ing pmem
> directly into userspace and exporting space on shared storage over
> pNFS.  Some day we might use the same mechanism for the similar things
> that RDMA does, or the swapfile code since that's essentially how it
> works today.
> 
> The other part of these file leases is that the filesystem revokes them
> any time it wants to perform a mapping operation on a file.  This breaks
> my mental model of how leases work, and if you commit to this for ext4
> then I'll have to remember that leases are different between xfs and
> ext4.  Worse, since the reason for skipping ext4_break_layouts seems to
> be the implementation detail that "DAX won't care", then someone else
> wiring up pNFS/future RDMA/whatever will also have to remember to put it
> back into ext4 or else kaboom.
> 
> Granted, Dave said all these things already, but I actually feel
> strongly enough to reiterate.

Jan, would you like me to call ext4_break_layouts() in ext4_insert_range() to
keep the lease mechanism consistent between ext4 and XFS, or would you prefer
the s/ext4_break_layouts()/ext4_dax_unmap_pages()/ rename?