[PATCH 3/4] mm/thp: allow drop THP from page cache

[Song Liu <songliubraving@fb.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Once a THP is added to the page cache, it cannot be dropped via
/proc/sys/vm/drop_caches. Fix this issue with proper handling in
invalidate_mapping_pages() and __remove_mapping().

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 mm/truncate.c | 12 ++++++++++++
 mm/vmscan.c   |  3 ++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 8563339041f6..dd9ebc1da356 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -592,6 +592,16 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
 					unlock_page(page);
 					continue;
 				}
+
+				/* Take a pin outside pagevec */
+				get_page(page);
+
+				/*
+				 * Drop extra pins before trying to invalidate
+				 * the huge page.
+				 */
+				pagevec_remove_exceptionals(&pvec);
+				pagevec_release(&pvec);
 			}
 
 			ret = invalidate_inode_page(page);
@@ -602,6 +612,8 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
 			 */
 			if (!ret)
 				deactivate_file_page(page);
+			if (PageTransHuge(page))
+				put_page(page);
 			count += ret;
 		}
 		pagevec_remove_exceptionals(&pvec);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c6659bb758a4..1d80a188ad4a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -932,7 +932,8 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
 	 * Note that if SetPageDirty is always performed via set_page_dirty,
 	 * and thus under the i_pages lock, then this ordering is not required.
 	 */
-	if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
+	if (unlikely(PageTransHuge(page)) &&
+			(PageSwapCache(page) || !PageSwapBacked(page)))
 		refcount = 1 + HPAGE_PMD_NR;
 	else
 		refcount = 2;
-- 
2.17.1