On 11/05/18 at 01:38pm, Michal Hocko wrote:
On Mon 05-11-18 18:25:20, Baoquan He wrote:
Hi Michal,
On 11/05/18 at 10:28am, Michal Hocko wrote:
Or something like this. Ugly as hell, no question about that. I also have to think about this some more to convince myself this will not result in an endless loop under some situations.
It failed. Paste the log and patch diff here, please help check if I made any mistake on manual code change. The log is at bottom.
The retry patch is obviously still racy, it just makes the race window slightly smaller and I hoped it would catch most of those races but this is obviously not the case.
I was thinking about your MIGRATE_MOVABLE check some more and I still do not like it much, we just change migrate type at many places and I have hard time to actually see this is always safe wrt. to what we need here.
We should be able to restore the zone type check though. The primary problem fixed by 15c30bc09085 ("mm, memory_hotplug: make has_unmovable_pages more robust") was that early allocations made it to the zone_movable range. If we add the check _after_ the PageReserved() check then we should be able to rule all bootmem allocation out.
So what about the following (on top of the previous patch which makes sense on its own I believe).
Yes, I think this looks very reasonable and should be robust.
Have tested it, hot removing 4 hotpluggable nodes continusously succeeds, and then hot adding them back, still works well.
So please feel free to add my Tested-by or Acked-by.
Tested-by: Baoquan He bhe@redhat.com or Acked-by: Baoquan He bhe@redhat.com
Thanks, Michal.
From d7ffd1342529c892f1de8999c3a5609211599c9d Mon Sep 17 00:00:00 2001 From: Michal Hocko mhocko@suse.com Date: Mon, 5 Nov 2018 13:28:51 +0100 Subject: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages
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@redhat.com Signed-off-by: Michal Hocko mhocko@suse.com
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