Message-ID: <dbfbd982-27f3-0d72-49e0-d3dd5fe636a8@bytedance.com>
Date:   Tue, 14 Feb 2023 19:26:23 +0800
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Subject: Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes
Content-Language: en-US
To:     David Hildenbrand <david@redhat.com>,
        Qi Zheng <arch0.zheng@gmail.com>,
        Mike Rapoport <rppt@kernel.org>
Cc:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Teng Hu <huteng.ht@bytedance.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Oscar Salvador <osalvador@suse.de>,
        Muchun Song <muchun.song@linux.dev>
References: <20230212110305.93670-1-zhengqi.arch@bytedance.com>
 <2484666e-e78e-549d-e075-b2c39d460d71@suse.cz>
 <85af4ada-96c8-1f99-90fa-9b6d63d0016e@bytedance.com>
 <c6908bd0-5f76-47eb-2b77-ce8f7bf9fbff@suse.cz>
 <eefc40e5-a14b-22c5-3480-6786afa1c8f4@redhat.com>
 <Y+tXrK/g1Nrd/q1h@kernel.org>
 <67240e55-af49-f20a-2b4b-b7d574cd910d@gmail.com>
 <22f0e262-982e-ea80-e52a-a3c924b31d58@redhat.com>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <22f0e262-982e-ea80-e52a-a3c924b31d58@redhat.com>
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On 2023/2/14 19:22, David Hildenbrand wrote:
> On 14.02.23 11:26, Qi Zheng wrote:
>>
>>
>> On 2023/2/14 17:43, Mike Rapoport wrote:
>>> On Tue, Feb 14, 2023 at 10:17:03AM +0100, David Hildenbrand wrote:
>>>> On 14.02.23 09:42, Vlastimil Babka wrote:
>>>>> On 2/13/23 12:00, Qi Zheng wrote:
>>>>>>
>>>>>>
>>>>>> On 2023/2/13 16:47, Vlastimil Babka wrote:
>>>>>>> On 2/12/23 12:03, Qi Zheng wrote:
>>>>>>>> In x86, numa_register_memblks() is only interested in
>>>>>>>> those nodes which have enough memory, so it skips over
>>>>>>>> all nodes with memory below NODE_MIN_SIZE (treated as
>>>>>>>> a memoryless node). Later on, we will initialize these
>>>>>>>> memoryless nodes (allocate pgdat in free_area_init()
>>>>>>>> and build zonelist etc), and will online these nodes
>>>>>>>> in init_cpu_to_node() and init_gi_nodes().
>>>>>>>>
>>>>>>>> After boot, these memoryless nodes are in N_ONLINE
>>>>>>>> state but not in N_MEMORY state. But we can still allocate
>>>>>>>> pages from these memoryless nodes.
>>>>>>>>
>>>>>>>> In SLUB, we only process nodes in the N_MEMORY state,
>>>>>>>> such as allocating their struct kmem_cache_node. So if
>>>>>>>> we allocate a page from the memoryless node above to
>>>>>>>> SLUB, the struct kmem_cache_node of the node corresponding
>>>>>>>> to this page is NULL, which will cause panic.
>>>>>>>>
>>>>>>>> For example, if we use qemu to start a two numa node kernel,
>>>>>>>> one of the nodes has 2M memory (less than NODE_MIN_SIZE),
>>>>>>>> and the other node has 2G, then we will encounter the
>>>>>>>> following panic:
>>>>>>>>
>>>>>>>> [    0.149844] BUG: kernel NULL pointer dereference, address: 
>>>>>>>> 0000000000000000
>>>>>>>> [    0.150783] #PF: supervisor write access in kernel mode
>>>>>>>> [    0.151488] #PF: error_code(0x0002) - not-present page
>>>>>>>> <...>
>>>>>>>> [    0.156056] RIP: 0010:_raw_spin_lock_irqsave+0x22/0x40
>>>>>>>> <...>
>>>>>>>> [    0.169781] Call Trace:
>>>>>>>> [    0.170159]  <TASK>
>>>>>>>> [    0.170448]  deactivate_slab+0x187/0x3c0
>>>>>>>> [    0.171031]  ? bootstrap+0x1b/0x10e
>>>>>>>> [    0.171559]  ? preempt_count_sub+0x9/0xa0
>>>>>>>> [    0.172145]  ? kmem_cache_alloc+0x12c/0x440
>>>>>>>> [    0.172735]  ? bootstrap+0x1b/0x10e
>>>>>>>> [    0.173236]  bootstrap+0x6b/0x10e
>>>>>>>> [    0.173720]  kmem_cache_init+0x10a/0x188
>>>>>>>> [    0.174240]  start_kernel+0x415/0x6ac
>>>>>>>> [    0.174738]  secondary_startup_64_no_verify+0xe0/0xeb
>>>>>>>> [    0.175417]  </TASK>
>>>>>>>> [    0.175713] Modules linked in:
>>>>>>>> [    0.176117] CR2: 0000000000000000
>>>>>>>>
>>>>>>>> In addition, we can also encountered this panic in the actual
>>>>>>>> production environment. We set up a 2c2g container with two
>>>>>>>> numa nodes, and then reserved 128M for kdump, and then we
>>>>>>>> can encountered the above panic in the kdump kernel.
>>>>>>>>
>>>>>>>> To fix it, we can filter memoryless nodes when allocating
>>>>>>>> pages.
>>>>>>>>
>>>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>>>>>> Reported-by: Teng Hu <huteng.ht@bytedance.com>
>>>>>>>
>>>>>>> Well AFAIK the key mechanism to only allocate from "good" nodes 
>>>>>>> is the
>>>>>>> zonelist, we shouldn't need to start putting extra checks like 
>>>>>>> this. So it
>>>>>>> seems to me that the code building the zonelists should take the
>>>>>>> NODE_MIN_SIZE constraint in mind.
>>>>>>
>>>>>> Indeed. How about the following patch:
>>>>>
>>>>> +Cc also David, forgot earlier.
>>>>>
>>>>> Looks good to me, at least.
>>>>>
>>>>>> @@ -6382,8 +6378,11 @@ int find_next_best_node(int node, nodemask_t
>>>>>> *used_node_mask)
>>>>>>             int min_val = INT_MAX;
>>>>>>             int best_node = NUMA_NO_NODE;
>>>>>>
>>>>>> -       /* Use the local node if we haven't already */
>>>>>> -       if (!node_isset(node, *used_node_mask)) {
>>>>>> +       /*
>>>>>> +        * Use the local node if we haven't already. But for 
>>>>>> memoryless
>>>>>> local
>>>>>> +        * node, we should skip it and fallback to other nodes.
>>>>>> +        */
>>>>>> +       if (!node_isset(node, *used_node_mask) && node_state(node,
>>>>>> N_MEMORY)) {
>>>>>>                     node_set(node, *used_node_mask);
>>>>>>                     return node;
>>>>>>             }
>>>>>>
>>>>>> For memoryless node, we skip it and fallback to other nodes when
>>>>>> build its zonelists.
>>>>>>
>>>>>> Say we have node0 and node1, and node0 is memoryless, then:
>>>>>>
>>>>>> [    0.102400] Fallback order for Node 0: 1
>>>>>> [    0.102931] Fallback order for Node 1: 1
>>>>>>
>>>>>> In this way, we will not allocate pages from memoryless node0.
>>>>>>
>>>>
>>>> In offline_pages(), we'll first build_all_zonelists() to then
>>>> node_states_clear_node()->node_clear_state(node, N_MEMORY);
>>>>
>>>> So at least on the offlining path, we wouldn't detect it properly yet I
>>>> assume, and build a zonelist that contains a now-memory-less node?
>>>
>>> Another question is what happens if a new memory is plugged into a node
>>> that had < NODE_MIN_SIZE of memory and after hotplug it stops being
>>> "memoryless".
>>
>> When going online and offline a memory will re-call
>> build_all_zonelists() to re-establish the zonelists (the zonelist of
>> itself and other nodes). So it can stop being "memoryless"
>> automatically.
>>
>> But in online_pages(), did not see the check of < NODE_MIN_SIZE.
> 
> TBH, this is the first time I hear of NODE_MIN_SIZE and it seems to be a 
> pretty x86 specific thing.
> 
> Are we sure we want to get NODE_MIN_SIZE involved?

Maybe add an arch_xxx() to handle it?

> 

-- 
Thanks,
Qi