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, 22 insertions, 2 deletions

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)