Re: [RFC PATCH] mm, memory_hotplug: do not clear numa_node association after hot_remove

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/08/2018 03:59 PM, Michal Hocko wrote:
> [Removing Wen Congyang and Tang Chen from the CC list because their
>  emails bounce. It seems that we will never learn about their motivation]
> 
> On Thu 08-11-18 11:04:13, Michal Hocko wrote:
>> From: Michal Hocko <mhocko@suse.com>
>>
>> Per-cpu numa_node provides a default node for each possible cpu. The
>> association gets initialized during the boot when the architecture
>> specific code explores cpu->NUMA affinity. When the whole NUMA node is
>> removed though we are clearing this association
>>
>> try_offline_node
>>   check_and_unmap_cpu_on_node
>>     unmap_cpu_on_node
>>       numa_clear_node
>>         numa_set_node(cpu, NUMA_NO_NODE)
>>
>> This means that whoever calls cpu_to_node for a cpu associated with such
>> a node will get NUMA_NO_NODE. This is problematic for two reasons. First
>> it is fragile because __alloc_pages_node would simply blow up on an
>> out-of-bound access. We have encountered this when loading kvm module
>> BUG: unable to handle kernel paging request at 00000000000021c0
>> IP: [<ffffffff8119ccb3>] __alloc_pages_nodemask+0x93/0xb70
>> PGD 800000ffe853e067 PUD 7336bbc067 PMD 0
>> Oops: 0000 [#1] SMP
>> [...]
>> CPU: 88 PID: 1223749 Comm: modprobe Tainted: G        W          4.4.156-94.64-default #1
>> task: ffff88727eff1880 ti: ffff887354490000 task.ti: ffff887354490000
>> RIP: 0010:[<ffffffff8119ccb3>]  [<ffffffff8119ccb3>] __alloc_pages_nodemask+0x93/0xb70
>> RSP: 0018:ffff887354493b40  EFLAGS: 00010202
>> RAX: 00000000000021c0 RBX: 0000000000000000 RCX: 0000000000000000
>> RDX: 0000000000000000 RSI: 0000000000000002 RDI: 00000000014000c0
>> RBP: 00000000014000c0 R08: ffffffffffffffff R09: 0000000000000000
>> R10: ffff88fffc89e790 R11: 0000000000014000 R12: 0000000000000101
>> R13: ffffffffa0772cd4 R14: ffffffffa0769ac0 R15: 0000000000000000
>> FS:  00007fdf2f2f1700(0000) GS:ffff88fffc880000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00000000000021c0 CR3: 00000077205ee000 CR4: 0000000000360670
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Stack:
>>  0000000000000086 014000c014d20400 ffff887354493bb8 ffff882614d20f4c
>>  0000000000000000 0000000000000046 0000000000000046 ffffffff810ac0c9
>>  ffff88ffe78c0000 ffffffff0000009f ffffe8ffe82d3500 ffff88ff8ac55000
>> Call Trace:
>>  [<ffffffffa07476cd>] alloc_vmcs_cpu+0x3d/0x90 [kvm_intel]
>>  [<ffffffffa0772c0c>] hardware_setup+0x781/0x849 [kvm_intel]
>>  [<ffffffffa04a1c58>] kvm_arch_hardware_setup+0x28/0x190 [kvm]
>>  [<ffffffffa04856fc>] kvm_init+0x7c/0x2d0 [kvm]
>>  [<ffffffffa0772cf2>] vmx_init+0x1e/0x32c [kvm_intel]
>>  [<ffffffff8100213a>] do_one_initcall+0xca/0x1f0
>>  [<ffffffff81193886>] do_init_module+0x5a/0x1d7
>>  [<ffffffff81112083>] load_module+0x1393/0x1c90
>>  [<ffffffff81112b30>] SYSC_finit_module+0x70/0xa0
>>  [<ffffffff8161cbc3>] entry_SYSCALL_64_fastpath+0x1e/0xb7
>> DWARF2 unwinder stuck at entry_SYSCALL_64_fastpath+0x1e/0xb7
>>
>> on an older kernel but the code is basically the same in the current
>> Linus tree as well. alloc_vmcs_cpu could use alloc_pages_nodemask which
>> would recognize NUMA_NO_NODE and use alloc_pages_node which would translate
>> it to numa_mem_id but that is wrong as well because it would use a cpu
>> affinity of the local CPU which might be quite far from the original node.

But then the original node is getting/already off-lined. The allocation is
going to come from a different node. alloc_pages_node() at least steer the
allocation alway from VM_BUG_ON() because of NUMA_NO_NODE by replacing it
with numa_mem_id().

If node fallback order is important for this allocation then could not it
use __alloc_pages_nodemask() directly giving preference for its zonelist
node and nodemask. Just curious.

>> It is also reasonable to expect that cpu_to_node will provide a sane value
>> and there might be many more callers like that.

AFAICS there are two choices here. Either mark them NUMA_NO_NODE for all
cpus of a node going offline or keep the existing mapping in case the node
comes back again.

>>
>> The second problem is that __register_one_node relies on cpu_to_node
>> to properly associate cpus back to the node when it is onlined. We do
>> not want to lose that link as there is no arch independent way to get it
>> from the early boot time AFAICS.

Retaining the links seems to be right unless unmap_cpu_on_node() is sort
of a weak callback letting arch to decide.

>>
>> Drop the whole check_and_unmap_cpu_on_node machinery and keep the
>> association to fix both issues. The NODE_DATA(nid) is not deallocated
Though retaining the link is a problem in itself but the allocation related
crash could be solved by exploring __alloc_pages_nodemask() options.

>> so it will stay in place and if anybody wants to allocate from that node
>> then a fallback node will be used.

Right, NODE_DATA(nid) is an advantage of retaining the link.