Re: [PATCH 2/3] mm: page_isolation: remove unnecessary freepage_migratetype check for unused page

From: Vlastimil Babka
Date: Tue Dec 09 2014 - 04:24:37 EST

On 12/09/2014 08:51 AM, Weijie Yang wrote:
when we test the pages in a range is free or not, there is a little
chance we encounter some page which is not in buddy but page_count is 0.
That means that page could be in the page-freeing path but not in the
buddy freelist, such as in pcplist

This shouldn't happen anymore IMHO. The pageblock is marked as MIGRATE_ISOLATE and then a lru+pcplist drain is performed. Nothing should be left on pcplist - anything newly freed goes directly to free lists. Hm, maybe it could be on lru cache, but that holds a page reference IIRC, so this test won't pass.

or wait for the zone->lock which the
tester is holding.

That could maybe happen, but is it worth testing? If yes, please add it in a comment to the code.

Back to the freepage_migratetype, we use it for a cached value for decide
which free-list the page go when freeing page. If the pageblock is isolated
the page will go to free-list[MIGRATE_ISOLATE] even if the cached type is
not MIGRATE_ISOLATE, the commit ad53f92e(fix incorrect isolation behavior
by rechecking migratetype) patch series have ensure this.

So the freepage_migratetype check for page_count==0 page in
__test_page_isolated_in_pageblock() is meaningless.
This patch removes the unnecessary freepage_migratetype check.

Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
mm/page_isolation.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 6e5174d..f7c9183 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -223,8 +223,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
page = pfn_to_page(pfn);
if (PageBuddy(page))
pfn += 1 << page_order(page);
- else if (page_count(page) == 0 &&
- get_freepage_migratetype(page) == MIGRATE_ISOLATE)
+ else if (page_count(page) == 0)
pfn += 1;
else if (skip_hwpoisoned_pages && PageHWPoison(page)) {

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