linux-kernel - [PATCH 4.2 107/110] thp: use is_zero_pfn() only after pte

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Message-Id: <20151106191709.547519749@linuxfoundation.org>
Date:	Fri,  6 Nov 2015 11:19:54 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.2 107/110] thp: use is_zero_pfn() only after pte_present() check

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

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

From: Minchan Kim <minchan@...nel.org>

commit 47aee4d8e314384807e98b67ade07f6da476aa75 upstream.

Use is_zero_pfn() on pteval only after pte_present() check on pteval
(It might be better idea to introduce is_zero_pte() which checks
pte_present() first).

Otherwise when working on a 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 unlucky, the application
segfaults and finally you could see below message on exit:

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

Fixes: ca0984caa823 ("mm: incorporate zero pages into transparent huge pages")
Signed-off-by: Minchan Kim <minchan@...nel.org>
Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Mel Gorman <mgorman@...e.de>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Rik van Riel <riel@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/huge_memory.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2132,7 +2132,8 @@ static int __collapse_huge_page_isolate(
 	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 (++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else

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