Messages in this thread | | | Date | Tue, 14 Feb 2023 12:29:42 +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 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@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?
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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