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

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 <[e-mail]>
Cc: Jin Dongming <[e-mail]>
Reviewed-by: Andrea Arcangeli <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Hidetoshi Seto <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page-flags.h

···
 	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 PageTransCompound(struct page *page)
+{
+	return 0;
+}
+
+static inline int PageTransTail(struct page *page)
 {
 	return 0;
 }

mm/memory-failure.c

···
 	 * 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)) {