linux-kernel - Re: [PATCH v2 1/2] mm: Uncharge poisoned pages

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Date:   Fri, 28 Apr 2017 09:31:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        Andi Kleen <andi@...stfloor.org>
Cc:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v2 1/2] mm: Uncharge poisoned pages

[CC Johannes and Vladimir - the patch is
http://lkml.kernel.org/r/1493130472-22843-2-git-send-email-ldufour@linux.vnet.ibm.com]

On Fri 28-04-17 08:07:55, Michal Hocko wrote:
> On Thu 27-04-17 13:51:23, Andi Kleen wrote:
> > Michal Hocko <mhocko@...nel.org> writes:
> > 
> > > On Tue 25-04-17 16:27:51, Laurent Dufour wrote:
> > >> When page are poisoned, they should be uncharged from the root memory
> > >> cgroup.
> > >> 
> > >> This is required to avoid a BUG raised when the page is onlined back:
> > >> BUG: Bad page state in process mem-on-off-test  pfn:7ae3b
> > >> page:f000000001eb8ec0 count:0 mapcount:0 mapping:          (null)
> > >> index:0x1
> > >> flags: 0x3ffff800200000(hwpoison)
> > >
> > > My knowledge of memory poisoning is very rudimentary but aren't those
> > > pages supposed to leak and never come back? In other words isn't the
> > > hoplug code broken because it should leave them alone?
> > 
> > Yes that would be the right interpretation. If it was really offlined
> > due to a hardware error the memory will be poisoned and any access
> > could cause a machine check.
> 
> OK, thanks for the clarification. Then I am not sure the patch is
> correct. Why do we need to uncharge that page at all?

Now, I have realized that we actually want to uncharge that page because
it will pin the memcg and we do not want to have that memcg and its
whole hierarchy pinned as well. This used to work before the charge
rework 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") I guess
because we used to uncharge on page cache removal.

I do not think the patch is correct, though. memcg_kmem_enabled() will
check whether kmem accounting is enabled and we are talking about page
cache pages here. You should be using mem_cgroup_uncharge instead.
-- 
Michal Hocko
SUSE Labs