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

Dave Chinner
2018-07-05T23:29:34Z

On Thu, Jul 05, 2018 at 01:40:17PM -0700, Dan Williams wrote:
> On Wed, Jul 4, 2018 at 8:59 PM, Darrick J. Wong <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> 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.
>
> This patch kit is only for the DAX fix, this isn't full layout lease
> support. Even XFS is special casing unmap with the BREAK_UNMAP flag.
> So ext4 is achieving feature parity for BREAK_UNMAP, just not
> BREAK_WRITE, yet.

BREAK_UNMAP is issued unconditionally by XFS for all fallocate
operations. There is no special except for extent shifting (up or
down) in XFS as this patch set is making for ext4. IOWs, this
patchset does not implement BREAK_UNMAP with the same semantics as
XFS.

Cheers,

Dave.
--
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org