[PATCH] thp: use is_zero_pfn only after pte_present check

[Minchan Kim]

Use is_zero_pfn on pteval only after pte_present check on pteval
(It might be better idea to introduce is_zero_pte where checks
pte_present first). Otherwise, it could work with swap or
migration entry and if pte_pfn's result is equal to zero_pfn
by chance, we lose user's data in __collapse_huge_page_copy.
So if you're luck, the application is segfaulted and finally you
could see below message when the application is exit.

BUG: Bad rss-counter state mm:ffff88007f099300 idx:2 val:3

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4b06b8db9df2..bbac913f96bc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2206,7 +2206,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
 	     _pte++, address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
-		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+		if (pte_none(pteval) || (pte_present(pteval) &&
+				is_zero_pfn(pte_pfn(pteval)))) {
 			if (!userfaultfd_armed(vma) &&
 			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
-- 
1.9.1


In khugepaged_scan_pmd, although there is no is_swap_pte check in
v4.2, we don't need to check pte_present check right before is_zero_pfn
because that part is just scanning operation so even if something wrong
happens rarely, it should filter out in __collapse_huge_page_isolate
with this patch.

In __collapse_huge_page_copy, we don't need the check, either.
Because every ptes in the vma's 2M area point out isolated LRU pages
and zero page so any pages couldn't be swap-out.

Thanks for the review.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/