From: Michal Hocko mhocko@suse.com
Baoquan He has noticed that 15c30bc09085 ("mm, memory_hotplug: make has_unmovable_pages more robust") is causing memory offlining failures on a movable node. After a further debugging it turned out that has_unmovable_pages fails prematurely because it stumbles over off-LRU pages. Nevertheless those pages are not on LRU because they are waiting on the pcp LRU caches (an example of __dump_page added by a debugging patch) [ 560.923297] page:ffffea043f39fa80 count:1 mapcount:0 mapping:ffff880e5dce1b59 index:0x7f6eec459 [ 560.931967] flags: 0x5fffffc0080024(uptodate|active|swapbacked) [ 560.937867] raw: 005fffffc0080024 dead000000000100 dead000000000200 ffff880e5dce1b59 [ 560.945606] raw: 00000007f6eec459 0000000000000000 00000001ffffffff ffff880e43ae8000 [ 560.953323] page dumped because: hotplug [ 560.957238] page->mem_cgroup:ffff880e43ae8000 [ 560.961620] has_unmovable_pages: pfn:0x10fd030d, found:0x1, count:0x0 [ 560.968127] page:ffffea043f40c340 count:2 mapcount:0 mapping:ffff880e2f2d8628 index:0x0 [ 560.976104] flags: 0x5fffffc0000006(referenced|uptodate) [ 560.981401] raw: 005fffffc0000006 dead000000000100 dead000000000200 ffff880e2f2d8628 [ 560.989119] raw: 0000000000000000 0000000000000000 00000002ffffffff ffff88010a8f5000 [ 560.996833] page dumped because: hotplug
The issue could be worked around by calling lru_add_drain_all but we can do better than that. We know that all swap backed pages are migrateable and the same applies for pages which do implement the migratepage callback.
Reported-by: Baoquan He bhe@redhat.com Fixes: 15c30bc09085 ("mm, memory_hotplug: make has_unmovable_pages more robust") Cc: stable Signed-off-by: Michal Hocko mhocko@suse.com ---
Hi, we have been discussing issue reported by Baoquan [1] mostly off-list and he has confirmed the patch solved failures he is seeing. I believe that has_unmovable_pages begs for a much better implementation and/or substantial pages isolation design rethinking but let's close the bug which can be really annoying first.
[1] http://lkml.kernel.org/r/20181101091055.GA15166@MiWiFi-R3L-srv
mm/page_alloc.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 863d46da6586..48ceda313332 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -7824,8 +7824,22 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count, if (__PageMovable(page)) continue;
- if (!PageLRU(page)) - found++; + if (PageLRU(page)) + continue; + + /* + * Some LRU pages might be temporarily off-LRU for all + * sort of different reasons - reclaim, migration, + * per-cpu LRU caches etc. + * Make sure we do not consider those pages to be unmovable. + */ + if (PageSwapBacked(page)) + continue; + + if (page->mapping && page->mapping->a_ops && + page->mapping->a_ops->migratepage) + continue; + /* * If there are RECLAIMABLE pages, we need to check * it. But now, memory offline itself doesn't call @@ -7839,7 +7853,7 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count, * is set to both of a memory hole page and a _used_ kernel * page at boot. */ - if (found > count) + if (++found > count) goto unmovable; } return false;