[Devel] [PATCH rh7] mm: vmscan: never isolate more pages than necessary

[Vladimir Davydov]

====
Along with "[PATCH rh7] mm: vmscan: use proportional scanning during
direct reclaim and full scan at DEF_PRIORITY" this should fix

https://jira.sw.ru/browse/PSBM-35275

I submitted this patch upstream (https://lkml.org/lkml/2015/8/3/404) and
it was merged into the mmotm tree. Hopefully, it will get merged into
Linus's tree soon.
====

If transparent huge pages are enabled, we can isolate many more pages
than we actually need to scan, because we count both single and huge
pages equally in isolate_lru_pages().

Since commit 5bc7b8aca942d ("mm: thp: add split tail pages to shrink
page list in page reclaim"), we scan all the tail pages immediately
after a huge page split (see shrink_page_list()). As a result, we can
reclaim up to SWAP_CLUSTER_MAX * HPAGE_PMD_NR (512 MB) in one run!

This is easy to catch on memcg reclaim with zswap enabled. The latter
makes swapout instant so that if we happen to scan an unreferenced huge
page we will evict both its head and tail pages immediately, which is
likely to result in excessive reclaim.

Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
---
 mm/vmscan.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a2c25b0fb0f8..831aab1ba5cf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1297,7 +1297,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	unsigned long nr_taken = 0;
 	unsigned long scan;
 
-	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
+	for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
+					!list_empty(src); scan++) {
 		struct page *page;
 		int nr_pages;
 
-- 
2.1.4