[PATCH v2 3/5] mm/memory-failure: give up error handling for non-tail-refcounted thp

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

"non anonymous thp" case is still racy with freeing thp, which causes panic
due to put_page() for refcount-0 page. It seems that closing up this race
might be hard (and/or not worth doing,) so let's give up the error handling
for this case.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
v1 -> v2:
- keep pr_err() message
---
 mm/memory-failure.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git v4.2-rc4.orig/mm/memory-failure.c v4.2-rc4/mm/memory-failure.c
index f72d2fad0b90..cd985530f102 100644
--- v4.2-rc4.orig/mm/memory-failure.c
+++ v4.2-rc4/mm/memory-failure.c
@@ -909,6 +909,18 @@ int get_hwpoison_page(struct page *page)
 	 * directly for tail pages.
 	 */
 	if (PageTransHuge(head)) {
+		/*
+		 * Non anonymous thp exists only in allocation/free time. We
+		 * can't handle such a case correctly, so let's give it up.
+		 * This should be better than triggering BUG_ON when kernel
+		 * tries to touch the "partially handled" page.
+		 */
+		if (!PageAnon(head)) {
+			pr_err("MCE: %#lx: non anonymous thp\n",
+				page_to_pfn(page));
+			return 0;
+		}
+
 		if (get_page_unless_zero(head)) {
 			if (PageTail(page))
 				get_page(page);
@@ -1134,15 +1146,6 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
 	}
 
 	if (!PageHuge(p) && PageTransHuge(hpage)) {
-		if (!PageAnon(hpage)) {
-			pr_err("MCE: %#lx: non anonymous thp\n", pfn);
-			if (TestClearPageHWPoison(p))
-				atomic_long_sub(nr_pages, &num_poisoned_pages);
-			put_page(p);
-			if (p != hpage)
-				put_page(hpage);
-			return -EBUSY;
-		}
 		if (unlikely(split_huge_page(hpage))) {
 			pr_err("MCE: %#lx: thp split failed\n", pfn);
 			if (TestClearPageHWPoison(p))
-- 
2.4.3

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