1 files changed, 46 insertions, 0 deletions

diff --git a/mm/internal.h b/mm/internal.h
index d071d380fb49..2189af491783 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -37,6 +37,52 @@ static inline void __put_page(struct page *page)
         atomic_dec(&page->_count);
 }
 
+static inline void __get_page_tail_foll(struct page *page,
+                                        bool get_page_head)
+{
+        /*
+         * If we're getting a tail page, the elevated page->_count is
+         * required only in the head page and we will elevate the head
+         * page->_count and tail page->_mapcount.
+         *
+         * We elevate page_tail->_mapcount for tail pages to force
+         * page_tail->_count to be zero at all times to avoid getting
+         * false positives from get_page_unless_zero() with
+         * speculative page access (like in
+         * page_cache_get_speculative()) on tail pages.
+         */
+        VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
+        VM_BUG_ON(atomic_read(&page->_count) != 0);
+        VM_BUG_ON(page_mapcount(page) < 0);
+        if (get_page_head)
+                atomic_inc(&page->first_page->_count);
+        atomic_inc(&page->_mapcount);
+}
+
+/*
+ * This is meant to be called as the FOLL_GET operation of
+ * follow_page() and it must be called while holding the proper PT
+ * lock while the pte (or pmd_trans_huge) is still mapping the page.
+ */
+static inline void get_page_foll(struct page *page)
+{
+        if (unlikely(PageTail(page)))
+                /*
+                 * This is safe only because
+                 * __split_huge_page_refcount() can't run under
+                 * get_page_foll() because we hold the proper PT lock.
+                 */
+                __get_page_tail_foll(page, true);
+        else {
+                /*
+                 * Getting a normal page or the head of a compound page
+                 * requires to already have an elevated page->_count.
+                 */
+                VM_BUG_ON(atomic_read(&page->_count) <= 0);
+                atomic_inc(&page->_count);
+        }
+}
+
 extern unsigned long highest_memmap_pfn;
 
 /*