Re: [PATCH v3] mm/memory_hotplug: refrain from adding memory into an impossible node

[David Hildenbrand <david@redhat.com>]

On 15.04.20 09:39, David Hildenbrand wrote:
> On 15.04.20 01:58, Vishal Verma wrote:
>> A misbehaving qemu created a situation where the ACPI SRAT table
>> advertised one fewer proximity domains than intended. The NFIT table did
>> describe all the expected proximity domains. This caused the device dax
>> driver to assign an impossible target_node to the device, and when
>> hotplugged as system memory, this would fail with the following
>> signature:
>>
>>   [  +0.001627] BUG: kernel NULL pointer dereference, address: 0000000000000088
>>   [  +0.001331] #PF: supervisor read access in kernel mode
>>   [  +0.000975] #PF: error_code(0x0000) - not-present page
>>   [  +0.000976] PGD 80000001767d4067 P4D 80000001767d4067 PUD 10e0c4067 PMD 0
>>   [  +0.001338] Oops: 0000 [#1] SMP PTI
>>   [  +0.000676] CPU: 4 PID: 22737 Comm: kswapd3 Tainted: G           O      5.6.0-rc5 #9
>>   [  +0.001457] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>>       BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
>>   [  +0.001990] RIP: 0010:prepare_kswapd_sleep+0x7c/0xc0
>>   [  +0.000780] Code: 89 df e8 87 fd ff ff 89 c2 31 c0 84 d2 74 e6 0f 1f 44
>>                       00 00 48 8b 05 fb af 7a 01 48 63 93 88 1d 01 00 48 8b
>> 		      84 d0 20 0f 00 00 <48> 3b 98 88 00 00 00 75 28 f0 80 a0
>> 		      80 00 00 00 fe f0 80 a3 38 20
>>   [  +0.002877] RSP: 0018:ffffc900017a3e78 EFLAGS: 00010202
>>   [  +0.000805] RAX: 0000000000000000 RBX: ffff8881209e0000 RCX: 0000000000000000
>>   [  +0.001115] RDX: 0000000000000003 RSI: 0000000000000000 RDI: ffff8881209e0e80
>>   [  +0.001098] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000008000
>>   [  +0.001092] R10: 0000000000000000 R11: 0000000000000003 R12: 0000000000000003
>>   [  +0.001092] R13: 0000000000000003 R14: 0000000000000000 R15: ffffc900017a3ec8
>>   [  +0.001091] FS:  0000000000000000(0000) GS:ffff888318c00000(0000) knlGS:0000000000000000
>>   [  +0.001275] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>   [  +0.000882] CR2: 0000000000000088 CR3: 0000000120b50002 CR4: 00000000001606e0
>>   [  +0.001095] Call Trace:
>>   [  +0.000388]  kswapd+0x103/0x520
>>   [  +0.000494]  ? finish_wait+0x80/0x80
>>   [  +0.000547]  ? balance_pgdat+0x5a0/0x5a0
>>   [  +0.000607]  kthread+0x120/0x140
>>   [  +0.000508]  ? kthread_create_on_node+0x60/0x60
>>   [  +0.000706]  ret_from_fork+0x3a/0x50
>>
>> Add a check in the add_memory path to ensure that the node to which we
>> are adding memory is in the node_possible_map
>>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
>> ---
>>  mm/memory_hotplug.c | 28 ++++++++++++++++++++++++++++
>>  1 file changed, 28 insertions(+)
>>
>> v2:
>> - Centralize the check in the add_memory path (David)
>> - Instead of failing, add the memory to a nearby node, while warning
>>   (and tainting) to call out attention to the firmware bug (Dan)
>>
>> v3:
>> - Fix the CONFIG_NUMA=n case, and use node 0 as the final fallback (Dan)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 0a54ffac8c68..536a809d6ebb 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -980,6 +980,30 @@ static int check_hotplug_memory_range(u64 start, u64 size)
>>  	return 0;
>>  }
>>  
>> +/*
>> + * Check that the node provided for adding memory was valid.
>> + * If not, find the nearest valid node and add the memory there while
>> + * tainting the kernel and displaying a warning to bring attention to the
>> + * underlying firmware problem.
>> + * Return nid if valid, or an adjusted node number that can be used instead
>> + * if the original nid was not valid
>> + */
> 
> -ETOOMUCHDOCUMENTAION
> 
> "If the given node cannot be used (!node_possible()), return the nearest
> possible node and WARN_TAINT() about firmware issues."
> 
>> +static int check_hotplug_node(int nid)
>> +{
>> +	int alt_nid;
>> +
>> +	if (node_possible(nid))
>> +		return nid;
>> +
>> +	alt_nid = numa_map_to_online_node(nid);
>> +	if (alt_nid == NUMA_NO_NODE)
>> +		alt_nid = first_online_node;
>> +	WARN_TAINT(1, TAINT_FIRMWARE_WORKAROUND,
>> +		   "node %d expected, but was absent from the node_possible_map, using %d instead\n",
>> +		   nid, alt_nid);
>> +	return alt_nid;
>> +}
>> +
>>  static int online_memory_block(struct memory_block *mem, void *arg)
>>  {
>>  	return device_online(&mem->dev);
>> @@ -1005,6 +1029,10 @@ int __ref add_memory_resource(int nid, struct resource *res)
>>  	if (ret)
>>  		return ret;
>>  
>> +	nid = check_hotplug_node(nid);
>> +	if (nid < 0)
>> +		return -ENXIO;
>> +
>>  	mem_hotplug_begin();
>>  
>>  	/*
>>
> 
> You should do the same on the memory removal path.

(I do wonder if the result could be different
(numa_map_to_online_node()/first_online_node)) on the removal path, and
if we should bail out when removing instead. Sounds better to me, adding
memory is more important in this case.

-- 
Thanks,

David / dhildenb