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    SubjectRe: [PATCH v6 12/15] xfs: require mmap lock for xfs_break_layouts()
    On Mon, Mar 19, 2018 at 11:19 AM, Darrick J. Wong
    <darrick.wong@oracle.com> wrote:
    > 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@oracle.com> 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. :)

    Of course, but then fixing up xfs_iolock_two_inodes_and_break_layout()
    is a follow on consideration from this patch series.

    > 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?

    Yes, I think so, trigger xfs_break_dax_layouts() on the destination
    inode on any write fault.

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