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> --- 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 -- 2.20.1
