[PATCH] thp: use is_zero_pfn after pte_present check

[Minchan Kim <minchan@kernel.org>]

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@kernel.org>
---

I found this bug with MADV_FREE hard test. Sometime, I saw
"Bad rss-counter" message with MM_SWAPENTS but it's really
rare, once a day if I was luck or once in five days if I was
unlucky so I am doing test still and just pass a few days but
I hope it will fix the issue.

 mm/huge_memory.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4b06b8db9df2..349590aa4533 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2665,15 +2665,25 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 	for (_address = address, _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)) {
 			if (!userfaultfd_armed(vma) &&
 			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else
 				goto out_unmap;
 		}
+
 		if (!pte_present(pteval))
 			goto out_unmap;
+
+		if (is_zero_pfn(pte_pfn(pteval))) {
+			if (!userfaultfd_armed(vma) &&
+			    ++none_or_zero <= khugepaged_max_ptes_none)
+				continue;
+			else
+				goto out_unmap;
+		}
+
 		if (pte_write(pteval))
 			writable = true;
 
-- 
1.9.1

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