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

[Michal Hocko]

On Fri 09-11-18 09:12:09, Anshuman Khandual wrote:
> 
> 
> 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@xxxxxxxx>
> >>
> >> 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.

How does the caller get the right node to allocate from? We do have the
proper zone list for the offline node so why not use it?

> >> 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.

Or update the mapping to the closeses node. I have chosen to keep the
mapping because it is the easiest and the most natural one.

> >> 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.

Yes that is the case but this looks like a very fragile fix to me. If
you are getting a node number from cpu_to_node then you shouldn't really
think about obscurities like which allocation function to use, right?
You should just get a valid node number.

Do you see any problems with the patch as is?
-- 
Michal Hocko
SUSE Labs