1 files changed, 53 insertions, 30 deletions

diff --git a/mm/swap.c b/mm/swap.c
index 3a442f18b0b3..87627f181c3f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -78,39 +78,22 @@ static void put_compound_page(struct page *page)
 {
         if (unlikely(PageTail(page))) {
                 /* __split_huge_page_refcount can run under us */
-                struct page *page_head = page->first_page;
-                smp_rmb();
-                /*
-                 * If PageTail is still set after smp_rmb() we can be sure
-                 * that the page->first_page we read wasn't a dangling pointer.
-                 * See __split_huge_page_refcount() smp_wmb().
-                 */
-                if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
+                struct page *page_head = compound_trans_head(page);
+
+                if (likely(page != page_head &&
+                           get_page_unless_zero(page_head))) {
                         unsigned long flags;
                         /*
-                         * Verify that our page_head wasn't converted
-                         * to a a regular page before we got a
-                         * reference on it.
+                         * page_head wasn't a dangling pointer but it
+                         * may not be a head page anymore by the time
+                         * we obtain the lock. That is ok as long as it
+                         * can't be freed from under us.
                          */
-                        if (unlikely(!PageHead(page_head))) {
-                                /* PageHead is cleared after PageTail */
-                                smp_rmb();
-                                VM_BUG_ON(PageTail(page));
-                                goto out_put_head;
-                        }
-                        /*
-                         * Only run compound_lock on a valid PageHead,
-                         * after having it pinned with
-                         * get_page_unless_zero() above.
-                         */
-                        smp_mb();
-                        /* page_head wasn't a dangling pointer */
                         flags = compound_lock_irqsave(page_head);
                         if (unlikely(!PageTail(page))) {
                                 /* __split_huge_page_refcount run before us */
                                 compound_unlock_irqrestore(page_head, flags);
                                 VM_BUG_ON(PageHead(page_head));
-                        out_put_head:
                                 if (put_page_testzero(page_head))
                                         __put_single_page(page_head);
                         out_put_single:
@@ -121,16 +104,17 @@ static void put_compound_page(struct page *page)
                         VM_BUG_ON(page_head != page->first_page);
                         /*
                          * We can release the refcount taken by
-                         * get_page_unless_zero now that
-                         * split_huge_page_refcount is blocked on the
-                         * compound_lock.
+                         * get_page_unless_zero() now that
+                         * __split_huge_page_refcount() is blocked on
+                         * the compound_lock.
                          */
                         if (put_page_testzero(page_head))
                                 VM_BUG_ON(1);
                         /* __split_huge_page_refcount will wait now */
-                        VM_BUG_ON(atomic_read(&page->_count) <= 0);
-                        atomic_dec(&page->_count);
+                        VM_BUG_ON(page_mapcount(page) <= 0);
+                        atomic_dec(&page->_mapcount);
                         VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
+                        VM_BUG_ON(atomic_read(&page->_count) != 0);
                         compound_unlock_irqrestore(page_head, flags);
                         if (put_page_testzero(page_head)) {
                                 if (PageHead(page_head))
@@ -160,6 +144,45 @@ void put_page(struct page *page)
 }
 EXPORT_SYMBOL(put_page);
 
+/*
+ * This function is exported but must not be called by anything other
+ * than get_page(). It implements the slow path of get_page().
+ */
+bool __get_page_tail(struct page *page)
+{
+        /*
+         * This takes care of get_page() if run on a tail page
+         * returned by one of the get_user_pages/follow_page variants.
+         * get_user_pages/follow_page itself doesn't need the compound
+         * lock because it runs __get_page_tail_foll() under the
+         * proper PT lock that already serializes against
+         * split_huge_page().
+         */
+        unsigned long flags;
+        bool got = false;
+        struct page *page_head = compound_trans_head(page);
+
+        if (likely(page != page_head && get_page_unless_zero(page_head))) {
+                /*
+                 * page_head wasn't a dangling pointer but it
+                 * may not be a head page anymore by the time
+                 * we obtain the lock. That is ok as long as it
+                 * can't be freed from under us.
+                 */
+                flags = compound_lock_irqsave(page_head);
+                /* here __split_huge_page_refcount won't run anymore */
+                if (likely(PageTail(page))) {
+                        __get_page_tail_foll(page, false);
+                        got = true;
+                }
+                compound_unlock_irqrestore(page_head, flags);
+                if (unlikely(!got))
+                        put_page(page_head);
+        }
+        return got;
+}
+EXPORT_SYMBOL(__get_page_tail);
+
 /**
  * put_pages_list() - release a list of pages
  * @pages: list of pages threaded on page->lru