Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break_layouts()

[Darrick J. Wong]

On Mon, Mar 19, 2018 at 10:57:55AM -0700, Dan Williams wrote:
> On Mon, Mar 19, 2018 at 10:33 AM, Darrick J. Wong
> <darrick.wong@xxxxxxxxxx> wrote:
> > On Thu, Mar 15, 2018 at 08:52:29AM -0700, Dan Williams wrote:
> >> In preparation for adding coordination between truncate operations and
> >> busy dax-pages, extend xfs_break_layouts() to assume it must be called
> >> with the mmap lock held. This locking scheme will be required for
> >> coordinating the break of 'dax layouts' (non-idle dax (ZONE_DEVICE)
> >> pages mapped into the file's address space).
> >
> > If I'm reading this right, you've added a requirement (for xfs anyway)
> > that we have to have grabbed MMAPLOCK_EXCL before calling break_layout()
> > so that the layout breaking process will block until active dmas have
> > finished.
> >
> > In 4.16 we added xfs_iolock_two_inodes_and_break_layout (in
> > xfs_reflink.c) to break pnfs leases for files that are about to be
> > reflinked (since pnfs and reflink aren't compatible either).  I think
> > that function will have to be adapted to take the appropriate mmap locks
> > too -- definitely the exclusive mmap lock for the destination file
> > because we anticipate punching out blocks.  I'm not sure about the
> > source file; I think taking the shared mmap lock is fine for that?
> 
> I don't see anything to adapt with respect to mmap locks since reflink
> and dax are mutually exclusive. The new lock coordination and layout
> breaking in xfs_break_dax_layouts() only applies to dax vs truncate,
> so xfs_iolock_two_inodes_and_break_layout() looks ok to me as is.

I was aiming for DAX + reflink someday working together.  :)

As far as that goes, I think the only thing we really have to do is
figure out how to make iomap_begin do the cow operation before letting
userspace write to (or MAP_SYNC a writable region) pmem, right?

--D

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