thp: keep huge zero page pinned until tlb flush

Andrea has found[1] a race condition on MMU-gather based TLB flush vs
split_huge_page() or shrinker which frees huge zero under us (patch 1/2
and 2/2 respectively).

With new THP refcounting, we don't need patch 1/2: mmu_gather keeps the
page pinned until flush is complete and the pin prevents the page from
being split under us.

We still need patch 2/2.  This is simplified version of Andrea's patch.
We don't need fancy encoding.

[1] http://lkml.kernel.org/r/1447938052-22165-1-git-send-email-aarcange@redhat.com

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 86f9f8b82f8e..5346de05f471 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -232,7 +232,7 @@ retry:
         return READ_ONCE(huge_zero_page);
 }
 
-static void put_huge_zero_page(void)
+void put_huge_zero_page(void)
 {
         /*
          * Counter should never go to zero here. Only shrinker can put
@@ -1684,12 +1684,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
         if (vma_is_dax(vma)) {
                 spin_unlock(ptl);
                 if (is_huge_zero_pmd(orig_pmd))
-                        put_huge_zero_page();
+                        tlb_remove_page(tlb, pmd_page(orig_pmd));
         } else if (is_huge_zero_pmd(orig_pmd)) {
                 pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
                 atomic_long_dec(&tlb->mm->nr_ptes);
                 spin_unlock(ptl);
-                put_huge_zero_page();
+                tlb_remove_page(tlb, pmd_page(orig_pmd));
         } else {
                 struct page *page = pmd_page(orig_pmd);
                 page_remove_rmap(page, true);