[PATCH] mm,thp: stop leaking unreleased file pages

[Hugh Dickins <hughd@xxxxxxxxxx>]

When collapse_file() calls try_to_release_page(), it has already
isolated the page: so if releasing buffers happens to fail (as it
sometimes does), remember to putback_lru_page(): otherwise that page is
left unreclaimable and unfreeable, and the file extent uncollapsible.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Cc: stable@xxxxxxxxxxxxxxx # v5.4+
---

 mm/khugepaged.c |    1 +
 1 file changed, 1 insertion(+)

--- 5.7-rc6/mm/khugepaged.c	2020-04-12 16:24:37.710999073 -0700
+++ linux/mm/khugepaged.c	2020-05-10 17:06:21.788398646 -0700
@@ -1692,6 +1692,7 @@ static void collapse_file(struct mm_stru
 		if (page_has_private(page) &&
 		    !try_to_release_page(page, GFP_KERNEL)) {
 			result = SCAN_PAGE_HAS_PRIVATE;
+			putback_lru_page(page);
 			goto out_unlock;
 		}