[PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages

[Michal Hocko]

Page state checks are racy. Under a heavy memory workload (e.g. stress
-m 200 -t 2h) it is quite easy to hit a race window when the page is
allocated but its state is not fully populated yet. A debugging patch to
dump the struct page state shows
: [  476.575516] has_unmovable_pages: pfn:0x10dfec00, found:0x1, count:0x0
: [  476.582103] page:ffffea0437fb0000 count:1 mapcount:1 mapping:ffff880e05239841 index:0x7f26e5000 compound_mapcount: 1
: [  476.592645] flags: 0x5fffffc0090034(uptodate|lru|active|head|swapbacked)

Note that the state has been checked for both PageLRU and PageSwapBacked
already. Closing this race completely would require some sort of retry
logic. This can be tricky and error prone (think of potential endless
or long taking loops).

Workaround this problem for movable zones at least. Such a zone should
only contain movable pages. 15c30bc09085 ("mm, memory_hotplug: make
has_unmovable_pages more robust") has told us that this is not strictly
true though. Bootmem pages should be marked reserved though so we can
move the original check after the PageReserved check. Pages from other
zones are still prone to races but we even do not pretend that memory
hotremove works for those so pre-mature failure doesn't hurt that much.

Reported-by: Baoquan He <bhe@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/page_alloc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 48ceda313332..5b64c5bc6ea0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7788,6 +7788,14 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 		if (PageReserved(page))
 			goto unmovable;
 
+		/*
+		 * If the zone is movable and we have ruled out all reserved
+		 * pages then it should be reasonably safe to assume the rest
+		 * is movable.
+		 */
+		if (zone_idx(zone) == ZONE_MOVABLE)
+			continue;
+
 		/*
 		 * Hugepages are not in LRU lists, but they're movable.
 		 * We need not scan over tail pages bacause we don't
-- 
2.19.1

-- 
Michal Hocko
SUSE Labs