Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes

[Qi Zheng <arch0.zheng@gmail.com>]

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.

> 
>> -- 
>> Thanks,
>>
>> David / dhildenb
>>
> 

-- 
Thanks,
Qi