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

[David Hildenbrand]

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?

--
Thanks,

David / dhildenb