linux-kernel - Re: [PATCH] mm: memcg: fix NULL pointer in mem_cgroup_track_foreign

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Date:   Mon, 30 Jan 2023 20:20:16 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Shakeel Butt <shakeelb@...gle.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Ma Wupeng <mawupeng1@...wei.com>, <shy828301@...il.com>
Subject: Re: [PATCH] mm: memcg: fix NULL pointer in
 mem_cgroup_track_foreign_dirty()



On 2023/1/30 16:48, Michal Hocko wrote:
> On Mon 30-01-23 09:16:13, Kefeng Wang wrote:
>>
>>
>> On 2023/1/30 5:48, Andrew Morton wrote:
>>> On Sun, 29 Jan 2023 10:44:51 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:
>>>
>>>> As commit 18365225f044 ("hwpoison, memcg: forcibly uncharge LRU pages"),
>>>
>>> Merged in 2017.
>>>
>>>> hwpoison will forcibly uncharg a LRU hwpoisoned page, the folio_memcg
>>>> could be NULl, then, mem_cgroup_track_foreign_dirty_slowpath() could
>>>> occurs a NULL pointer dereference, let's do not record the foreign
>>>> writebacks for folio memcg is null in mem_cgroup_track_foreign() to
>>>> fix it.
>>>>
>>>> Reported-by: Ma Wupeng <mawupeng1@...wei.com>
>>>> Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
>>>
>>> Merged in 2019.
>>>
...
> 
> Just to make sure I understand. The page has been hwpoisoned, uncharged
> but stayed in the page cache so a next page fault on the address has blowned
> up?
> 
> Say we address the NULL memcg case. What is the resulting behavior?
> Doesn't userspace access a poisoned page and get a silend memory
> corruption?

+ Yang Shi

Check previous link[1], seems that it is a known issue, and there is a 
TODO list for storage backed filesystems from Yang.


[1] 
https://lore.kernel.org/all/20211020210755.23964-6-shy828301@gmail.com/T/#m1d40559ca2dcf94396df5369214288f69dec379b