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[14.202.194.140]) by smtp.gmail.com with ESMTPSA id x13-v6sm2141586pge.13.2018.11.07.04.53.33 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 07 Nov 2018 04:53:35 -0800 (PST) Date: Wed, 7 Nov 2018 23:53:24 +1100 From: Balbir Singh To: Michal Hocko Cc: Andrew Morton , Baoquan He , Oscar Salvador , linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages Message-ID: <20181107125324.GD9042@350D> References: <20181106095524.14629-1-mhocko@kernel.org> <20181106203518.GC9042@350D> <20181107073548.GU27423@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181107073548.GU27423@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 07, 2018 at 08:35:48AM +0100, Michal Hocko wrote: > On Wed 07-11-18 07:35:18, Balbir Singh wrote: > > On Tue, Nov 06, 2018 at 10:55:24AM +0100, Michal Hocko wrote: > > > From: 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-and-tested-by: Baoquan He > > > Acked-by: Baoquan He > > > Fixes: "mm, memory_hotplug: make has_unmovable_pages more robust") > > > Signed-off-by: Michal Hocko > > > --- > > > > > > Hi, > > > this has been reported [1] and we have tried multiple things to address > > > the issue. The only reliable way was to reintroduce the movable zone > > > check into has_unmovable_pages. This time it should be safe also for > > > the bug originally fixed by 15c30bc09085. > > > > > > [1] http://lkml.kernel.org/r/20181101091055.GA15166@MiWiFi-R3L-srv > > > mm/page_alloc.c | 8 ++++++++ > > > 1 file changed, 8 insertions(+) > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > index 863d46da6586..c6d900ee4982 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; > > > + > > > /* > > > > > > There is a WARN_ON() in case of failure at the end of the routine, > > is that triggered when we hit the bug? If we're adding this patch, > > the WARN_ON needs to go as well. > > No the warning should stay in case we encounter reserved pages in zone > movable. > Fair enough! > > The check seems to be quite aggressive and in a loop that iterates > > pages, but has nothing to do with the page, did you mean to make > > the check > > > > zone_idx(page_zone(page)) == ZONE_MOVABLE > > Does it make any difference? Can we actually encounter a page from a > different zone here? > Just to avoid page state related issues, do we want to go ahead with the migration if zone_idx(page_zone(page)) != ZONE_MOVABLE. > > it also skips all checks for pinned pages and other checks > > Yes, this is intentional and the comment tries to explain why. I wish we > could be add a more specific checks for movable pages - e.g. detect long > term pins that would prevent migration - but we do not have any facility > for that. Please note that the worst case of a false positive is a > repeated migration failure and user has a way to break out of migration > by a signal. > Basically isolate_pages() will fail as opposed to hotplug failing upfront. The basic assertion this patch makes is that all ZONE_MOVABLE pages that are not reserved are hotpluggable. Balbir Singh.