+ mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page.patch added to -mm tree

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The patch titled
     Subject: mm: thp: fix mmu_notifier in migrate_misplaced_transhuge_page()
has been added to the -mm tree.  Its filename is
     mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page.patch

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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: mm: thp: fix mmu_notifier in migrate_misplaced_transhuge_page()

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@xxxxxxxxxx
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/huge_memory.c |   14 +++++++++++++-
 mm/migrate.c     |   19 ++++++-------------
 2 files changed, 19 insertions(+), 14 deletions(-)

--- a/mm/huge_memory.c~mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page
+++ a/mm/huge_memory.c
@@ -1578,8 +1578,20 @@ vm_fault_t do_huge_pmd_numa_page(struct
 	 * We are not sure a pending tlb flush here is for a huge page
 	 * mapping or not. Hence use the tlb range variant
 	 */
-	if (mm_tlb_flush_pending(vma->vm_mm))
+	if (mm_tlb_flush_pending(vma->vm_mm)) {
 		flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
+		/*
+		 * change_huge_pmd() released the pmd lock before
+		 * invalidating the secondary MMUs sharing the primary
+		 * MMU pagetables (with ->invalidate_range()). The
+		 * mmu_notifier_invalidate_range_end() (which
+		 * internally calls ->invalidate_range()) in
+		 * change_pmd_range() will run after us, so we can't
+		 * rely on it here and we need an explicit invalidate.
+		 */
+		mmu_notifier_invalidate_range(vma->vm_mm, haddr,
+					      haddr + HPAGE_PMD_SIZE);
+	}
 
 	/*
 	 * Migrate the THP to the requested node, returns with page unlocked
--- a/mm/migrate.c~mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page
+++ a/mm/migrate.c
@@ -1975,8 +1975,8 @@ int migrate_misplaced_transhuge_page(str
 	int isolated = 0;
 	struct page *new_page = NULL;
 	int page_lru = page_is_file_cache(page);
-	unsigned long mmun_start = address & HPAGE_PMD_MASK;
-	unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;
+	unsigned long start = address & HPAGE_PMD_MASK;
+	unsigned long end = start + HPAGE_PMD_SIZE;
 
 	new_page = alloc_pages_node(node,
 		(GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
@@ -2003,11 +2003,9 @@ int migrate_misplaced_transhuge_page(str
 	WARN_ON(PageLRU(new_page));
 
 	/* Recheck the target PMD */
-	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
 	ptl = pmd_lock(mm, pmd);
 	if (unlikely(!pmd_same(*pmd, entry) || !page_ref_freeze(page, 2))) {
 		spin_unlock(ptl);
-		mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 
 		/* Reverse changes made by migrate_page_copy() */
 		if (TestClearPageActive(new_page))
@@ -2038,8 +2036,8 @@ int migrate_misplaced_transhuge_page(str
 	 * new page and page_add_new_anon_rmap guarantee the copy is
 	 * visible before the pagetable update.
 	 */
-	flush_cache_range(vma, mmun_start, mmun_end);
-	page_add_anon_rmap(new_page, vma, mmun_start, true);
+	flush_cache_range(vma, start, end);
+	page_add_anon_rmap(new_page, vma, start, true);
 	/*
 	 * At this point the pmd is numa/protnone (i.e. non present)
 	 * and the TLB has already been flushed globally. So no TLB
@@ -2052,7 +2050,7 @@ int migrate_misplaced_transhuge_page(str
 	 * at any given time, MADV_DONTNEED won't wait on the pmd lock
 	 * and it'll skip clearing this pmd.
 	 */
-	set_pmd_at(mm, mmun_start, pmd, entry);
+	set_pmd_at(mm, start, pmd, entry);
 	update_mmu_cache_pmd(vma, address, &entry);
 
 	page_ref_unfreeze(page, 2);
@@ -2061,11 +2059,6 @@ int migrate_misplaced_transhuge_page(str
 	set_page_owner_migrate_reason(new_page, MR_NUMA_MISPLACED);
 
 	spin_unlock(ptl);
-	/*
-	 * No need to double call mmu_notifier->invalidate_range() callback as
-	 * the above pmdp_huge_clear_flush_notify() did already call it.
-	 */
-	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 
 	/* Take an "isolate" reference and put new page on the LRU. */
 	get_page(new_page);
@@ -2089,7 +2082,7 @@ out_fail:
 	ptl = pmd_lock(mm, pmd);
 	if (pmd_same(*pmd, entry)) {
 		entry = pmd_modify(entry, vma->vm_page_prot);
-		set_pmd_at(mm, mmun_start, pmd, entry);
+		set_pmd_at(mm, start, pmd, entry);
 		update_mmu_cache_pmd(vma, address, &entry);
 	}
 	spin_unlock(ptl);
_

Patches currently in -mm which might be from aarcange@xxxxxxxxxx are

mm-thp-relax-__gfp_thisnode-for-madv_hugepage-mappings.patch
userfaultfd-allow-get_mempolicympol_f_nodempol_f_addr-to-trigger-userfaults.patch
mm-thp-fix-madv_dontneed-vs-migrate_misplaced_transhuge_page-race-condition.patch
mm-thp-fix-mmu_notifier-in-migrate_misplaced_transhuge_page.patch
mm-thp-relocate-flush_cache_range-in-migrate_misplaced_transhuge_page.patch




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