mm/memory.c: use entry = ACCESS_ONCE(*pte) in handle_pte_fault()

Use ACCESS_ONCE() in handle_pte_fault() when getting the entry or orig_pte
upon which all subsequent decisions and pte_same() tests will be made.

I have no evidence that its lack is responsible for the mm/filemap.c:202
BUG_ON(page_mapped(page)) in __delete_from_page_cache() found by trinity,
and I am not optimistic that it will fix it.  But I have found no other
explanation, and ACCESS_ONCE() here will surely not hurt.

If gcc does re-access the pte before passing it down, then that would be
disastrous for correct page fault handling, and certainly could explain
the page_mapped() BUGs seen (concurrent fault causing page to be mapped in
a second time on top of itself: mapcount 2 for a single pte).

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index d67fd9fcf1f2e11d8b77c513113475df125ad99d..87d2f3eeeb15d8cd23fe69c4c1142a5879dac40a 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3177,7 +3177,7 @@ static int handle_pte_fault(struct mm_struct *mm,
        pte_t entry;
        spinlock_t *ptl;
 
-       entry = *pte;
+       entry = ACCESS_ONCE(*pte);
        if (!pte_present(entry)) {
                if (pte_none(entry)) {
                        if (vma->vm_ops) {