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

[David Hildenbrand]

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@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".
>
> 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