[PATCH v2] mm/page_owner.c: remove redudant drain_all_pages

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

Remove an expensive and unnecessary operation as PCP pages are safely
skipped when reading page owner. PCP pages can be skipped because
PAGE_EXT_OWNER_ALLOCATED is cleared.

With draining PCP pages, these pages are moved to buddy list so they
can be identified as buddy pages and skipped quickly. Although it improved
efficiency of PFN walker, the drain is guaranteed expensive that is
unlikely to be offset by a slight increase in efficiency when skipping
free pages.

PAGE_EXT_OWNER_ALLOCATED is cleared in the page owner reset path below:
	free_unref_page
		-> free_unref_page_prepare
			-> free_pcp_prepare
				-> free_pages_prepare which do page owner
				reset
		-> free_unref_page_commit which add pages into pcp list

Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
---
 mm/page_owner.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 90023f9..54f3e03 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -524,8 +524,6 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
 		pfn++;
 
-	drain_all_pages(NULL);
-
 	/* Find an allocated page */
 	for (; pfn < max_pfn; pfn++) {
 		/*
-- 
2.7.4