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

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Date:   Tue, 7 Dec 2021 19:40:33 -0500
From:   Nico Pache <npache@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        shakeelb@...gle.com, ktkhai@...tuozzo.com, shy828301@...il.com,
        guro@...com, vbabka@...e.cz, vdavydov.dev@...il.com,
        raquini@...hat.com, mhocko@...e.com, david@...hat.com
Subject: Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker_info on
 offlined nodes



On 12/7/21 18:44, Andrew Morton 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).

Thanks for the review! I took your changes below into consideration for my V3.

My knowledge of offlined/onlined nodes is quite limited but after looking into
it it doesnt seem like anything clears the state of NODE_DATA(nid) after a
try_offline_node is attempted. So theoretically the panic we saw would not
happen. What is the expected behavior of trying to allocate a page on a offline
node?

> 
> 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?
> 
>> --- 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?
>