mm: migration: fix migration of huge PMD shared pages

The page migration code employs try_to_unmap() to try and unmap the source
page.  This is accomplished by using rmap_walk to find all vmas where the
page is mapped.  This search stops when page mapcount is zero.  For shared
PMD huge pages, the page map count is always 1 no matter the number of
mappings.  Shared mappings are tracked via the reference count of the PMD
page.  Therefore, try_to_unmap stops prematurely and does not completely
unmap all mappings of the source page.

This problem can result is data corruption as writes to the original
source page can happen after contents of the page are copied to the target
page.  Hence, data is lost.

This problem was originally seen as DB corruption of shared global areas
after a huge page was soft offlined due to ECC memory errors.  DB
developers noticed they could reproduce the issue by (hotplug) offlining
memory used to back huge pages.  A simple testcase can reproduce the
problem by creating a shared PMD mapping (note that this must be at least
PUD_SIZE in size and PUD_SIZE aligned (1GB on x86)), and using
migrate_pages() to migrate process pages between nodes while continually
writing to the huge pages being migrated.

To fix, have the try_to_unmap_one routine check for huge PMD sharing by
calling huge_pmd_unshare for hugetlbfs huge pages.  If it is a shared
mapping it will be 'unshared' which removes the page table entry and drops
the reference on the PMD page.  After this, flush caches and TLB.

mmu notifiers are called before locking page tables, but we can not be
sure of PMD sharing until page tables are locked.  Therefore, check for
the possibility of PMD sharing before locking so that notifiers can
prepare for the worst possible case.

Link: http://lkml.kernel.org/r/20180823205917.16297-2-mike.kravetz@oracle.com
[mike.kravetz@oracle.com: make _range_in_vma() a static inline]
  Link: http://lkml.kernel.org/r/6063f215-a5c8-2f0c-465a-2c515ddc952d@oracle.com
Fixes: 39dde65c9940 ("shared page table for hugetlb page")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 39 insertions, 3 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index eb477809a5c0..1e79fac3186b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
         }
 
         /*
-         * We have to assume the worse case ie pmd for invalidation. Note that
-         * the page can not be free in this function as call of try_to_unmap()
-         * must hold a reference on the page.
+         * For THP, we have to assume the worse case ie pmd for invalidation.
+         * For hugetlb, it could be much worse if we need to do pud
+         * invalidation in the case of pmd sharing.
+         *
+         * Note that the page can not be free in this function as call of
+         * try_to_unmap() must hold a reference on the page.
          */
         end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page)));
+        if (PageHuge(page)) {
+                /*
+                 * If sharing is possible, start and end will be adjusted
+                 * accordingly.
+                 */
+                adjust_range_if_pmd_sharing_possible(vma, &start, &end);
+        }
         mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
 
         while (page_vma_mapped_walk(&pvmw)) {
@@ -1409,6 +1419,32 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                 subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
                 address = pvmw.address;
 
+                if (PageHuge(page)) {
+                        if (huge_pmd_unshare(mm, &address, pvmw.pte)) {
+                                /*
+                                 * huge_pmd_unshare unmapped an entire PMD
+                                 * page.  There is no way of knowing exactly
+                                 * which PMDs may be cached for this mm, so
+                                 * we must flush them all.  start/end were
+                                 * already adjusted above to cover this range.
+                                 */
+                                flush_cache_range(vma, start, end);
+                                flush_tlb_range(vma, start, end);
+                                mmu_notifier_invalidate_range(mm, start, end);
+
+                                /*
+                                 * The ref count of the PMD page was dropped
+                                 * which is part of the way map counting
+                                 * is done for shared PMDs.  Return 'true'
+                                 * here.  When there is no other sharing,
+                                 * huge_pmd_unshare returns false and we will
+                                 * unmap the actual page and drop map count
+                                 * to zero.
+                                 */
+                                page_vma_mapped_walk_done(&pvmw);
+                                break;
+                        }
+                }
 
                 if (IS_ENABLED(CONFIG_MIGRATION) &&
                     (flags & TTU_MIGRATION) &&