thp: add compound_trans_head() helper

Cleanup some code with common compound_trans_head helper.

Signed-off-by: Andrea Arcangeli <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Marcelo Tosatti <[e-mail]>
Cc: Avi Kivity <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/huge_mm.h

···
 		return HPAGE_PMD_NR;
 	return 1;
 }
+static inline struct page *compound_trans_head(struct page *page)
+{
+	if (PageTail(page)) {
+		struct page *head;
+		head = page->first_page;
+		smp_rmb();
+		/*
+		 * head may be a dangling pointer.
+		 * __split_huge_page_refcount clears PageTail before
+		 * overwriting first_page, so if PageTail is still
+		 * there it means the head pointer isn't dangling.
+		 */
+		if (PageTail(page))
+			return head;
+	}
+	return page;
+}
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 #define HPAGE_PMD_SHIFT ({ BUG(); 0; })
 #define HPAGE_PMD_MASK ({ BUG(); 0; })
···
 	do { } while (0)
 #define wait_split_huge_page(__anon_vma, __pmd)	\
 	do { } while (0)
+#define compound_trans_head(page) compound_head(page)
 static inline int hugepage_madvise(struct vm_area_struct *vma,
 				   unsigned long *vm_flags, int advice)
 {

mm/ksm.c

···
 static struct page *page_trans_compound_anon(struct page *page)
 {
 	if (PageTransCompound(page)) {
-		struct page *head;
-		head = compound_head(page);
+		struct page *head = compound_trans_head(page);
 		/*
-		 * head may be a dangling pointer.
-		 * __split_huge_page_refcount clears PageTail
-		 * before overwriting first_page, so if
-		 * PageTail is still there it means the head
-		 * pointer isn't dangling.
+		 * head may actually be splitted and freed from under
+		 * us but it's ok here.
 		 */
-		if (head != page) {
-			smp_rmb();
-			if (!PageTransCompound(page))
-				return NULL;
-		}
 		if (PageAnon(head))
 			return head;
 	}

virt/kvm/kvm_main.c

···
 inline int kvm_is_mmio_pfn(pfn_t pfn)
 {
 	if (pfn_valid(pfn)) {
-		struct page *head;
+		int reserved;
 		struct page *tail = pfn_to_page(pfn);
-		head = compound_head(tail);
+		struct page *head = compound_trans_head(tail);
+		reserved = PageReserved(head);
 		if (head != tail) {
-			smp_rmb();
 			/*
-			 * head may be a dangling pointer.
-			 * __split_huge_page_refcount clears PageTail
-			 * before overwriting first_page, so if
-			 * PageTail is still there it means the head
-			 * pointer isn't dangling.
+			 * "head" is not a dangling pointer
+			 * (compound_trans_head takes care of that)
+			 * but the hugepage may have been splitted
+			 * from under us (and we may not hold a
+			 * reference count on the head page so it can
+			 * be reused before we run PageReferenced), so
+			 * we've to check PageTail before returning
+			 * what we just read.
 			 */
-			if (PageTail(tail)) {
-				/*
-				 * the "head" is not a dangling
-				 * pointer but the hugepage may have
-				 * been splitted from under us (and we
-				 * may not hold a reference count on
-				 * the head page so it can be reused
-				 * before we run PageReferenced), so
-				 * we've to recheck PageTail before
-				 * returning what we just read.
-				 */
-				int reserved = PageReserved(head);
-				smp_rmb();
-				if (PageTail(tail))
-					return reserved;
-			}
+			smp_rmb();
+			if (PageTail(tail))
+				return reserved;
 		}
 		return PageReserved(tail);
 	}