thp: alter compound get_page/put_page

Alter compound get_page/put_page to keep references on subpages too, in
order to allow __split_huge_page_refcount to split an hugepage even while
subpages have been pinned by one of the get_user_pages() variants.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 83 insertions, 7 deletions

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);