[PATCH] thp: allow a hwpoisoned head page to be put back to LRU

[Dean Nelson <dnelson@redhat.com>]

Andrea Arcangeli pointed out to me that a check in __memory_failure() which
was intended to prevent THP tail pages from being checked for the absence
of the PG_lru flag (something that is always the case), was also preventing
THP head pages from being checked.

A THP head page could actually benefit from the call to shake_page() by
ending up being put back to a LRU, provided it had been waiting in a
pagevec array.

Andrea suggested that the "!PageTransCompound(p)" in the if-statement
should be replaced by a "!PageTransTail(p)", thus allowing THP head pages
to be checked and possibly shaken.

Signed-off-by: Dean Nelson <dnelson@redhat.com>

---
 include/linux/page-flags.h |   20 ++++++++++++++++++++
 mm/memory-failure.c        |    2 +-
 2 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e90a673..ca8b626 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -414,11 +414,26 @@ static inline int PageTransHuge(struct page *page)
 	return PageHead(page);
 }
 
+/*
+ * PageTransCompound returns true for both transparent huge pages
+ * and hugetlbfs pages, so it should only be called when it's known
+ * that hugetlbfs pages aren't involved.
+ */
 static inline int PageTransCompound(struct page *page)
 {
 	return PageCompound(page);
 }
 
+/*
+ * PageTransTail returns true for both transparent huge pages
+ * and hugetlbfs pages, so it should only be called when it's known
+ * that hugetlbfs pages aren't involved.
+ */
+static inline int PageTransTail(struct page *page)
+{
+	return PageTail(page);
+}
+
 #else
 
 static inline int PageTransHuge(struct page *page)
@@ -430,6 +445,11 @@ static inline int PageTransCompound(struct page *page)
 {
 	return 0;
 }
+
+static inline int PageTransTail(struct page *page)
+{
+	return 0;
+}
 #endif
 
 #ifdef CONFIG_MMU
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 56080ea..c22076f 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1063,7 +1063,7 @@ int __memory_failure(unsigned long pfn, int trapno, int flags)
 	 * The check (unnecessarily) ignores LRU pages being isolated and
 	 * walked by the page reclaim code, however that's not a big loss.
 	 */
-	if (!PageHuge(p) && !PageTransCompound(p)) {
+	if (!PageHuge(p) && !PageTransTail(p)) {
 		if (!PageLRU(p))
 			shake_page(p, 0);
 		if (!PageLRU(p)) {

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