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

[Qi Zheng <zhengqi.arch@bytedance.com>]

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:

@@ -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.

> 
>> ---
>>   mm/page_alloc.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 588555754601..b9cce56f4e21 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4188,6 +4188,11 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>>   			(alloc_flags & ALLOC_CPUSET) &&
>>   			!__cpuset_zone_allowed(zone, gfp_mask))
>>   				continue;
>> +
>> +		/* Don't allocate page from memoryless nodes. */
>> +		if (!node_state((zone_to_nid(zone)), N_MEMORY))
>> +			continue;
>> +
>>   		/*
>>   		 * When allocating a page cache page for writing, we
>>   		 * want to get it from a node that is within its dirty
> 

-- 
Thanks,
Qi