Messages in this thread | | | Date | Tue, 14 Feb 2023 12:22:58 +0100 | Subject | Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes | From | David Hildenbrand <> |
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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?
-- Thanks,
David / dhildenb
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