rmap: support file thp

Naive approach: on mapping/unmapping the page as compound we update
->_mapcount on each 4k page.  That's not efficient, but it's not obvious
how we can optimize this.  We can look into optimization later.

PG_double_map optimization doesn't work for file pages since lifecycle
of file pages is different comparing to anon pages: file page can be
mapped again at any time.

Link: http://lkml.kernel.org/r/1466021202-61880-11-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/mm/util.c b/mm/util.c
index b756ee3..8d010ef 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -412,6 +412,12 @@ int __page_mapcount(struct page *page)
 	int ret;
 
 	ret = atomic_read(&page->_mapcount) + 1;
+	/*
+	 * For file THP page->_mapcount contains total number of mapping
+	 * of the page: no need to look into compound_mapcount.
+	 */
+	if (!PageAnon(page) && !PageHuge(page))
+		return ret;
 	page = compound_head(page);
 	ret += atomic_read(compound_mapcount_ptr(page)) + 1;
 	if (PageDoubleMap(page))