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@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>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 25c7d75..25ef59b 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1558,8 +1558,20 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t pmd)
         * 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

diff --git a/mm/migrate.c b/mm/migrate.c
index 93d9a1e..905c226 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1975,8 +1975,8 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
        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(struct mm_struct *mm,
        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(struct mm_struct *mm,
         * 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 can be currently
@@ -2051,7 +2049,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
         * 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);
@@ -2060,11 +2058,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
        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);
@@ -2088,7 +2081,7 @@ 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);