thp: fix MADV_DONTNEED vs. numa balancing race

In case prot_numa, we are under down_read(mmap_sem).  It's critical to
not clear pmd intermittently to avoid race with MADV_DONTNEED which is
also under down_read(mmap_sem):

	CPU0:				CPU1:
				change_huge_pmd(prot_numa=1)
				 pmdp_huge_get_and_clear_notify()
madvise_dontneed()
 zap_pmd_range()
  pmd_trans_huge(*pmd) == 0 (without ptl)
  // skip the pmd
				 set_pmd_at();
				 // pmd is re-established

The race makes MADV_DONTNEED miss the huge pmd and don't clear it
which may break userspace.

Found by code analysis, never saw triggered.

Link: http://lkml.kernel.org/r/20170302151034.27829-3-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 33 insertions, 1 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a513861a9037..26769465af63 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1746,7 +1746,39 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
         if (prot_numa && pmd_protnone(*pmd))
                 goto unlock;
 
-        entry = pmdp_huge_get_and_clear_notify(mm, addr, pmd);
+        /*
+         * In case prot_numa, we are under down_read(mmap_sem). It's critical
+         * to not clear pmd intermittently to avoid race with MADV_DONTNEED
+         * which is also under down_read(mmap_sem):
+         *
+         *      CPU0:                           CPU1:
+         *                              change_huge_pmd(prot_numa=1)
+         *                               pmdp_huge_get_and_clear_notify()
+         * madvise_dontneed()
+         *  zap_pmd_range()
+         *   pmd_trans_huge(*pmd) == 0 (without ptl)
+         *   // skip the pmd
+         *                               set_pmd_at();
+         *                               // pmd is re-established
+         *
+         * The race makes MADV_DONTNEED miss the huge pmd and don't clear it
+         * which may break userspace.
+         *
+         * pmdp_invalidate() is required to make sure we don't miss
+         * dirty/young flags set by hardware.
+         */
+        entry = *pmd;
+        pmdp_invalidate(vma, addr, pmd);
+
+        /*
+         * Recover dirty/young flags.  It relies on pmdp_invalidate to not
+         * corrupt them.
+         */
+        if (pmd_dirty(*pmd))
+                entry = pmd_mkdirty(entry);
+        if (pmd_young(*pmd))
+                entry = pmd_mkyoung(entry);
+
         entry = pmd_modify(entry, newprot);
         if (preserve_write)
                 entry = pmd_mk_savedwrite(entry);