[PATCH 87/97] mm/memory.c: use entry = ACCESS_ONCE(*pte) in handle_pte_fault()

[Mel Gorman]

From: Hugh Dickins <hughd@xxxxxxxxxx>

commit c0d73261f5c1355a35b8b40e871d31578ce0c044 upstream.

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@xxxxxxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 99fe3aa..b870c9c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3698,7 +3698,7 @@ 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) {
-- 
1.8.4.5

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