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

Dan Williams
2018-07-06T05:08:33Z

On Thu, Jul 5, 2018 at 4:29 PM Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org> wrote:

> 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.

Ah true, I see that now.