[RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

filemap_map_pages takes a speculative reference to each page in the
range before it tries to lock that page. While this is correct it
also can influence page migration which will bail out when seeing
an elevated reference count. The faultaround code would bail on
seeing a locked page so we can pro-actively check the PageLocked
bit before page_cache_get_speculative and prevent from pointless
reference count churn.

Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/filemap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 81adec8ee02c..c76d6a251770 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2553,6 +2553,9 @@ void filemap_map_pages(struct vm_fault *vmf,
 			goto next;
 
 		head = compound_head(page);
+
+		if (PageLocked(head))
+			goto next;
 		if (!page_cache_get_speculative(head))
 			goto next;
 
-- 
2.19.1