4 files changed, 129 insertions, 9 deletions

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
index d7efdbf640c7..fec13200868f 100644
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -16,6 +16,16 @@
 
 #ifdef __HAVE_ARCH_PTE_SPECIAL
 
+static inline void get_huge_page_tail(struct page *page)
+{
+        /*
+         * __split_huge_page_refcount() cannot run
+         * from under us.
+         */
+        VM_BUG_ON(atomic_read(&page->_count) < 0);
+        atomic_inc(&page->_count);
+}
+
 /*
  * The performance critical leaf functions are made noinline otherwise gcc
  * inlines everything into a single function which results in too much
@@ -47,6 +57,8 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
                         put_page(page);
                         return 0;
                 }
+                if (PageTail(page))
+                        get_huge_page_tail(page);
                 pages[*nr] = page;
                 (*nr)++;
 
diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 738e6593799d..06f56fcf9a77 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -105,6 +105,16 @@ static inline void get_head_page_multiple(struct page *page, int nr)
         atomic_add(nr, &page->_count);
 }
 
+static inline void get_huge_page_tail(struct page *page)
+{
+        /*
+         * __split_huge_page_refcount() cannot run
+         * from under us.
+         */
+        VM_BUG_ON(atomic_read(&page->_count) < 0);
+        atomic_inc(&page->_count);
+}
+
 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
                 unsigned long end, int write, struct page **pages, int *nr)
 {
@@ -128,6 +138,8 @@ static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
         do {
                 VM_BUG_ON(compound_head(page) != head);
                 pages[*nr] = page;
+                if (PageTail(page))
+                        get_huge_page_tail(page);
                 (*nr)++;
                 page++;
                 refs++;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3b1754ad8785..a2718e1ed585 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -353,9 +353,29 @@ static inline int page_count(struct page *page)
 
 static inline void get_page(struct page *page)
 {
-        page = compound_head(page);
-        VM_BUG_ON(atomic_read(&page->_count) == 0);
+        /*
+         * Getting a normal page or the head of a compound page
+         * requires to already have an elevated page->_count. Only if
+         * we're getting a tail page, the elevated page->_count is
+         * required only in the head page, so for tail pages the
+         * bugcheck only verifies that the page->_count isn't
+         * negative.
+         */
+        VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
         atomic_inc(&page->_count);
+        /*
+         * Getting a tail page will elevate both the head and tail
+         * page->_count(s).
+         */
+        if (unlikely(PageTail(page))) {
+                /*
+                 * This is safe only because
+                 * __split_huge_page_refcount can't run under
+                 * get_page().
+                 */
+                VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
+                atomic_inc(&page->first_page->_count);
+        }
 }
 
 static inline struct page *virt_to_head_page(const void *x)
diff --git a/mm/swap.c b/mm/swap.c
index 3f4854205b16..33f5292fe132 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -56,17 +56,93 @@ static void __page_cache_release(struct page *page)
                 del_page_from_lru(zone, page);
                 spin_unlock_irqrestore(&zone->lru_lock, flags);
         }
+}
+
+static void __put_single_page(struct page *page)
+{
+        __page_cache_release(page);
         free_hot_cold_page(page, 0);
 }
 
-static void put_compound_page(struct page *page)
+static void __put_compound_page(struct page *page)
 {
-        page = compound_head(page);
-        if (put_page_testzero(page)) {
-                compound_page_dtor *dtor;
+        compound_page_dtor *dtor;
+
+        __page_cache_release(page);
+        dtor = get_compound_page_dtor(page);
+        (*dtor)(page);
+}
 
-                dtor = get_compound_page_dtor(page);
-                (*dtor)(page);
+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))) {
+                        unsigned long flags;
+                        /*
+                         * Verify that our page_head wasn't converted
+                         * to a a regular page before we got a
+                         * reference on it.
+                         */
+                        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:
+                                if (put_page_testzero(page))
+                                        __put_single_page(page);
+                                return;
+                        }
+                        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.
+                         */
+                        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(atomic_read(&page_head->_count) <= 0);
+                        compound_unlock_irqrestore(page_head, flags);
+                        if (put_page_testzero(page_head))
+                                __put_compound_page(page_head);
+                } else {
+                        /* page_head is a dangling pointer */
+                        VM_BUG_ON(PageTail(page));
+                        goto out_put_single;
+                }
+        } else if (put_page_testzero(page)) {
+                if (PageHead(page))
+                        __put_compound_page(page);
+                else
+                        __put_single_page(page);
         }
 }
 
@@ -75,7 +151,7 @@ void put_page(struct page *page)
         if (unlikely(PageCompound(page)))
                 put_compound_page(page);
         else if (put_page_testzero(page))
-                __page_cache_release(page);
+                __put_single_page(page);
 }
 EXPORT_SYMBOL(put_page);