Message-ID: <4386151c-0328-d207-9a71-933ef61817f9@redhat.com>
Date:   Tue, 14 Feb 2023 12:29:42 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>,
        Qi Zheng <arch0.zheng@...il.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Teng Hu <huteng.ht@...edance.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes

On 14.02.23 12:26, Qi Zheng wrote:
> 
> 
> 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@...edance.com>
>>>>>>>>> Reported-by: Teng Hu <huteng.ht@...edance.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?

I still haven't figured out what we want to achieve with NODE_MIN_SIZE 
at all. It smells like an arch-specific hack looking at

"Don't confuse VM with a node that doesn't have the minimum amount of 
memory"

Why shouldn't mm-core deal with that?

I'd appreciate an explanation of the bigger picture, what the issue is 
and what the approach to solve it is (including memory onlining/offlining).

-- 
Thanks,

David / dhildenb

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