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Violators will be prosecuted for from ; Wed, 3 May 2017 12:34:54 +0100 Subject: Re: [PATCH v2 1/2] mm: Uncharge poisoned pages References: <1493130472-22843-1-git-send-email-ldufour@linux.vnet.ibm.com> <1493130472-22843-2-git-send-email-ldufour@linux.vnet.ibm.com> <20170427143721.GK4706@dhcp22.suse.cz> <87pofxk20k.fsf@firstfloor.org> <20170428060755.GA8143@dhcp22.suse.cz> <20170428073136.GE8143@dhcp22.suse.cz> <3eb86373-dafc-6db9-82cd-84eb9e8b0d37@linux.vnet.ibm.com> <20170428134831.GB26705@dhcp22.suse.cz> <20170502185507.GB19165@dhcp22.suse.cz> From: Laurent Dufour Date: Wed, 3 May 2017 13:34:48 +0200 MIME-Version: 1.0 In-Reply-To: <20170502185507.GB19165@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <20041995-0b68-7471-6439-ef327329c9f8@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko , Andi Kleen , Johannes Weiner Cc: Naoya Horiguchi , linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, Vladimir Davydov On 02/05/2017 20:55, Michal Hocko wrote: > On Tue 02-05-17 16:59:30, Laurent Dufour wrote: >> On 28/04/2017 15:48, Michal Hocko wrote: > [...] >>> This is getting quite hairy. What is the expected page count of the >>> hwpoison page? > > OK, so from the quick check of the hwpoison code it seems that the ref > count will be > 1 (from get_hwpoison_page). > >>> I guess we would need to update the VM_BUG_ON in the >>> memcg uncharge code to ignore the page count of hwpoison pages if it can >>> be arbitrary. >> >> Based on the experiment I did, page count == 2 when isolate_lru_page() >> succeeds, even in the case of a poisoned page. > > that would make some sense to me. The page should have been already > unmapped therefore but memory_failure increases the ref count and 1 is > for isolate_lru_page(). > >> In my case I think this >> is because the page is still used by the process which is calling madvise(). >> >> I'm wondering if I'm looking at the right place. May be the poisoned >> page should remain attach to the memory_cgroup until no one is using it. >> In that case this means that something should be done when the page is >> off-lined... I've to dig further here. > > No, AFAIU the page will not drop the reference count down to 0 in most > cases. Maybe there are some scenarios where this can happen but I would > expect that the poisoned page will be mapped and in use most of the time > and won't drop down 0. And then we should really uncharge it because it > will pin the memcg and make it unfreeable which doesn't seem to be what > we want. So does the following work reasonable? Andi, Johannes, what do > you think? I cannot say I would be really comfortable touching hwpoison > code as I really do not understand the workflow. Maybe we want to move > this uncharge down to memory_failure() right before we report success? > --- > From 8bf0791bcf35996a859b6d33fb5494e5b53de49d Mon Sep 17 00:00:00 2001 > From: Michal Hocko > Date: Tue, 2 May 2017 20:32:24 +0200 > Subject: [PATCH] hwpoison, memcg: forcibly uncharge LRU pages > > Laurent Dufour has noticed that hwpoinsoned pages are kept charged. In > his particular case he has hit a bad_page("page still charged to cgroup") > when onlining a hwpoison page. While this looks like something that shouldn't > happen in the first place because onlining hwpages and returning them to > the page allocator makes only little sense it shows a real problem. > > hwpoison pages do not get freed usually so we do not uncharge them (at > least not since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API")). > Each charge pins memcg (since e8ea14cc6ead ("mm: memcontrol: take a css > reference for each charged page")) as well and so the mem_cgroup and the > associated state will never go away. Fix this leak by forcibly > uncharging a LRU hwpoisoned page in delete_from_lru_cache(). We also > have to tweak uncharge_list because it cannot rely on zero ref count > for these pages. > > Fixes: 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") > Reported-by: Laurent Dufour FWIW: Tested-by: Laurent Dufour > Signed-off-by: Michal Hocko > --- > mm/memcontrol.c | 2 +- > mm/memory-failure.c | 7 +++++++ > 2 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 16c556ac103d..4cf26059adb1 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -5527,7 +5527,7 @@ static void uncharge_list(struct list_head *page_list) > next = page->lru.next; > > VM_BUG_ON_PAGE(PageLRU(page), page); > - VM_BUG_ON_PAGE(page_count(page), page); > + VM_BUG_ON_PAGE(!PageHWPoison(page) && page_count(page), page); > > if (!page->mem_cgroup) > continue; > diff --git a/mm/memory-failure.c b/mm/memory-failure.c > index 8a6bd3a9eb1e..4497d9619bb4 100644 > --- a/mm/memory-failure.c > +++ b/mm/memory-failure.c > @@ -541,6 +541,13 @@ static int delete_from_lru_cache(struct page *p) > */ > ClearPageActive(p); > ClearPageUnevictable(p); > + > + /* > + * Poisoned page might never drop its ref count to 0 so we have to > + * uncharge it manually from its memcg. > + */ > + mem_cgroup_uncharge(p); > + > /* > * drop the page count elevated by isolate_lru_page() > */ > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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