linux-kernel - Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker

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Date:   Tue, 7 Dec 2021 19:33:40 -0500
From:   Nico Pache <npache@...hat.com>
To:     Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>, raquini@...hat.com,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker_info on
 offlined nodes



On 12/7/21 19:26, Yang Shi wrote:
> On Tue, Dec 7, 2021 at 3:44 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>>
>> On Tue,  7 Dec 2021 17:40:13 -0500 Nico Pache <npache@...hat.com> wrote:
>>
>>> We have run into a panic caused by a shrinker allocation being attempted
>>> on an offlined node.
>>>
>>> Our crash analysis has determined that the issue originates from trying
>>> to allocate pages on an offlined node in expand_one_shrinker_info. This
>>> function makes the incorrect assumption that we can allocate on any node.
>>> To correct this we make sure the node is online before tempting an
>>> allocation. If it is not online choose the closest node.
>>
>> This isn't fully accurate, is it?  We could allocate on a node which is
>> presently offline but which was previously onlined, by testing
>> NODE_DATA(nid).
>>
>> It isn't entirely clear to me from the v1 discussion why this approach
>> isn't being taken?
>>
>> AFAICT the proposed patch is *already* taking this approach, by having
>> no protection against a concurrent or subsequent node offlining?
> 
> AFAICT, we have not reached agreement on how to fix it yet. I saw 3
> proposals at least:
> 
> 1. From Michal, allocate node data for all possible nodes.
> https://lore.kernel.org/all/Ya89aqij6nMwJrIZ@dhcp22.suse.cz/T/#u
> 
> 2. What this patch does. Proposed originally from
> https://lore.kernel.org/all/20211108202325.20304-1-amakhalov@vmware.com/T/#u

Correct me if im wrong, but isn't that a different caller? This patch fixes the
issue in expand_one_shrinker_info.

> 3. From David, fix in node_zonelist().
> https://lore.kernel.org/all/51c65635-1dae-6ba4-daf9-db9df0ec35d8@redhat.com/T/#u
> 
>>
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -222,13 +222,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>>       int size = map_size + defer_size;
>>>
>>>       for_each_node(nid) {
>>> +             int tmp = nid;
>>
>> Not `tmp', please.  Better to use an identifier which explains the
>> variable's use.  target_nid?
>>
>> And a newline after defining locals, please.
>>
>>>               pn = memcg->nodeinfo[nid];
>>>               old = shrinker_info_protected(memcg, nid);
>>>               /* Not yet online memcg */
>>>               if (!old)
>>>                       return 0;
>>>
>>> -             new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
>>> +             if(!node_online(nid))
>>
>> s/if(/if (/
>>
>>> +                     tmp = numa_mem_id();
>>> +             new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, tmp);
>>>               if (!new)
>>>                       return -ENOMEM;
>>>
>>
>> And a code comment fully explaining what's going on here?
>