From: Andi Kleen <andi@firstfloor.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org
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>
Date: Thu, 27 Apr 2017 13:51:23 -0700
In-Reply-To: <20170427143721.GK4706@dhcp22.suse.cz> (Michal Hocko's message of
        "Thu, 27 Apr 2017 16:37:21 +0200")
Message-ID: <87pofxk20k.fsf@firstfloor.org>
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Michal Hocko <mhocko@kernel.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.

hwpoison has an own "unpoison" option (only used for debugging), which
I think handles this.

-Andi