[PATCH 3/7] khugepaged: Drain LRU add pagevec to get rid of extra pins

["Kirill A. Shutemov" <kirill@shutemov.name>]

__collapse_huge_page_isolate() may fail due to extra pin in the LRU add
pagevec. It's petty common for swapin case: we swap in pages just to
fail due to the extra pin.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 14d7afc90786..39e0994abeb8 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -585,11 +585,19 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		 * The page must only be referenced by the scanned process
 		 * and page swap cache.
 		 */
+		if (page_count(page) != 1 + PageSwapCache(page)) {
+			/*
+			 * Drain pagevec and retry just in case we can get rid
+			 * of the extra pin, like in swapin case.
+			 */
+			lru_add_drain();
+		}
 		if (page_count(page) != 1 + PageSwapCache(page)) {
 			unlock_page(page);
 			result = SCAN_PAGE_COUNT;
 			goto out;
 		}
+
 		if (pte_write(pteval)) {
 			writable = true;
 		} else {
-- 
2.26.0