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@xxxxxxxxxxxxx> > > > > > Reported-by: Teng Hu <huteng.ht@xxxxxxxxxxxxx> > > > > > > > > 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". > -- > Thanks, > > David / dhildenb > -- Sincerely yours, Mike.