change_huge_pmd() after arming the numa/protnone pmd doesn't flush the TLB
right away.  do_huge_pmd_numa_page() flushes the TLB before calling
migrate_misplaced_transhuge_page().  By the time do_huge_pmd_numa_page()
runs some CPU could still access the page through the TLB.

change_huge_pmd() before arming the numa/protnone transhuge pmd calls
mmu_notifier_invalidate_range_start().  So there's no need of
mmu_notifier_invalidate_range_start()/mmu_notifier_invalidate_range_only_end()
sequence in migrate_misplaced_transhuge_page() too, because by the time
migrate_misplaced_transhuge_page() runs, the pmd mapping has already been
invalidated in the secondary MMUs.  It has to or if a secondary MMU can
still write to the page, the migrate_page_copy() would lose data.

However an explicit mmu_notifier_invalidate_range() is needed before
migrate_misplaced_transhuge_page() starts copying the data of the
transhuge page or the below can happen for MMU notifier users sharing the
primary MMU pagetables and only implementing ->invalidate_range:

CPU0		CPU1		GPU sharing linux pagetables using
                                only ->invalidate_range
-----------	------------	---------
				GPU secondary MMU writes to the page
				mapped by the transhuge pmd
change_pmd_range()
mmu..._range_start()
->invalidate_range_start() noop
change_huge_pmd()
set_pmd_at(numa/protnone)
pmd_unlock()
		do_huge_pmd_numa_page()
		CPU TLB flush globally (1)
		CPU cannot write to page
		migrate_misplaced_transhuge_page()
				GPU writes to the page...
		migrate_page_copy()
				...GPU stops writing to the page
CPU TLB flush (2)
mmu..._range_end() (3)
->invalidate_range_stop() noop
->invalidate_range()
				GPU secondary MMU is invalidated
				and cannot write to the page anymore
				(too late)

Just like we need a CPU TLB flush (1) because the TLB flush (2) arrives
too late, we also need a mmu_notifier_invalidate_range() before calling
migrate_misplaced_transhuge_page(), because the ->invalidate_range() in
(3) also arrives too late.

This requirement is the result of the lazy optimization in
change_huge_pmd() that releases the pmd_lock without first flushing the
TLB and without first calling mmu_notifier_invalidate_range().

Even converting the removed mmu_notifier_invalidate_range_only_end() into
a mmu_notifier_invalidate_range_end() would not have been enough to fix
this, because it run after migrate_page_copy().

After the hugepage data copy is done migrate_misplaced_transhuge_page()
can proceed and call set_pmd_at without having to flush the TLB nor any
secondary MMUs because the secondary MMU invalidate, just like the CPU TLB
flush, has to happen before the migrate_page_copy() is called or it would
be a bug in the first place (and it was for drivers using
->invalidate_range()).

KVM is unaffected because it doesn't implement ->invalidate_range().

The standard PAGE_SIZEd migrate_misplaced_page is less accelerated and
uses the generic migrate_pages which transitions the pte from
numa/protnone to a migration entry in try_to_unmap_one() and flushes TLBs
and all mmu notifiers there before copying the page.

Link: http://lkml.kernel.org/r/20181013002430.698-3-aarcange@redhat.com


Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Aaron Tomlin <atomlin@redhat.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>