3.8.13.12 -stable review patch.  If anyone has any objections, please let me 
know.

------------------

From: Hugh Dickins <hu...@google.com>

commit 750e8165f5e87b6a142be953640eabb13a9d350a upstream.

Occasionally we hit the BUG_ON(pmd_trans_huge(*pmd)) at the end of
__split_huge_page_pmd(): seen when doing madvise(,,MADV_DONTNEED).

It's invalid: we don't always have down_write of mmap_sem there: a racing
do_huge_pmd_wp_page() might have copied-on-write to another huge page
before our split_huge_page() got the anon_vma lock.

Forget the BUG_ON, just go back and try again if this happens.

Signed-off-by: Hugh Dickins <hu...@google.com>
Acked-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
Cc: Andrea Arcangeli <aarca...@redhat.com>
Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
Cc: David Rientjes <rient...@google.com>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Kamal Mostafa <ka...@canonical.com>
---
 mm/huge_memory.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e9308d3..a057a7d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2740,6 +2740,7 @@ void __split_huge_page_pmd(struct vm_area_struct *vma, 
unsigned long address,
 
        mmun_start = haddr;
        mmun_end   = haddr + HPAGE_PMD_SIZE;
+again:
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
        spin_lock(&mm->page_table_lock);
        if (unlikely(!pmd_trans_huge(*pmd))) {
@@ -2762,7 +2763,14 @@ void __split_huge_page_pmd(struct vm_area_struct *vma, 
unsigned long address,
        split_huge_page(page);
 
        put_page(page);
-       BUG_ON(pmd_trans_huge(*pmd));
+
+       /*
+        * We don't always have down_write of mmap_sem here: a racing
+        * do_huge_pmd_wp_page() might have copied-on-write to another
+        * huge page before our split_huge_page() got the anon_vma lock.
+        */
+       if (unlikely(pmd_trans_huge(*pmd)))
+               goto again;
 }
 
 void split_huge_page_pmd_mm(struct mm_struct *mm, unsigned long address,
-- 
1.8.1.2

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