Re: [PATCH] mm: fix panic in __alloc_pages

[David Hildenbrand]

On 02.11.21 08:47, Michal Hocko wrote:
> [CC Oscar and David]
> 
> On Mon 01-11-21 13:13:12, Alexey Makhalov wrote:
>> There is a kernel panic caused by __alloc_pages() accessing
>> uninitialized NODE_DATA(nid). Uninitialized node data exists
>> during the time when CPU with memoryless node was added but
>> not onlined yet. Panic can be easy reproduced by disabling
>> udev rule for automatic onlining hot added CPU followed by
>> CPU with memoryless node hot add.
>>
>> This is a panic caused by percpu code doing allocations for
>> all possible CPUs and hitting this issue:
>>
>>  CPU2 has been hot-added
>>  BUG: unable to handle page fault for address: 0000000000001608
>>  #PF: supervisor read access in kernel mode
>>  #PF: error_code(0x0000) - not-present page
>>  PGD 0 P4D 0
>>  Oops: 0000 [#1] SMP PTI
>>  CPU: 0 PID: 1 Comm: systemd Tainted: G            E     5.15.0-rc7+ #11
>>  Hardware name: VMware, Inc. VMware7,1/440BX Desktop Reference Platform, BIOS VMW
>>
>>  RIP: 0010:__alloc_pages+0x127/0x290
> 
> Could you resolve this into a specific line of the source code please?
> 
>>  Code: 4c 89 f0 5b 41 5c 41 5d 41 5e 41 5f 5d c3 44 89 e0 48 8b 55 b8 c1 e8 0c 83 e0 01 88 45 d0 4c 89 c8 48 85 d2 0f 85 1a 01 00 00 <45> 3b 41 08 0f 82 10 01 00 00 48 89 45 c0 48 8b 00 44 89 e2 81 e2
>>  RSP: 0018:ffffc900006f3bc8 EFLAGS: 00010246
>>  RAX: 0000000000001600 RBX: 0000000000000000 RCX: 0000000000000000
>>  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000cc2
>>  RBP: ffffc900006f3c18 R08: 0000000000000001 R09: 0000000000001600
>>  R10: ffffc900006f3a40 R11: ffff88813c9fffe8 R12: 0000000000000cc2
>>  R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000cc2
>>  FS:  00007f27ead70500(0000) GS:ffff88807ce00000(0000) knlGS:0000000000000000
>>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>  CR2: 0000000000001608 CR3: 000000000582c003 CR4: 00000000001706b0
>>  Call Trace:
>>   pcpu_alloc_pages.constprop.0+0xe4/0x1c0
>>   pcpu_populate_chunk+0x33/0xb0
>>   pcpu_alloc+0x4d3/0x6f0
>>   __alloc_percpu_gfp+0xd/0x10
>>   alloc_mem_cgroup_per_node_info+0x54/0xb0
>>   mem_cgroup_alloc+0xed/0x2f0
>>   mem_cgroup_css_alloc+0x33/0x2f0
>>   css_create+0x3a/0x1f0
>>   cgroup_apply_control_enable+0x12b/0x150
>>   cgroup_mkdir+0xdd/0x110
>>   kernfs_iop_mkdir+0x4f/0x80
>>   vfs_mkdir+0x178/0x230
>>   do_mkdirat+0xfd/0x120
>>   __x64_sys_mkdir+0x47/0x70
>>   ? syscall_exit_to_user_mode+0x21/0x50
>>   do_syscall_64+0x43/0x90
>>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> Node can be in one of the following states:
>> 1. not present (nid == NUMA_NO_NODE)
>> 2. present, but offline (nid > NUMA_NO_NODE, node_online(nid) == 0,
>> 				NODE_DATA(nid) == NULL)
>> 3. present and online (nid > NUMA_NO_NODE, node_online(nid) > 0,
>> 				NODE_DATA(nid) != NULL)
>>
>> alloc_page_{bulk_array}node() functions verify for nid validity only
>> and do not check if nid is online. Enhanced verification check allows
>> to handle page allocation when node is in 2nd state.
> 
> I do not think this is a correct approach. We should make sure that the
> proper fallback node is used instead. This means that the zone list is
> initialized properly. IIRC this has been a problem in the past and it
> has been fixed. The initialization code is quite subtle though so it is
> possible that this got broken again.

I'm a little confused:

In add_memory_resource() we hotplug the new node if required and set it
online. Memory might get onlined later, via online_pages().

So after add_memory_resource()->__try_online_node() succeeded, we have
an online pgdat -- essentially 3.

This patch detects if we're past 3. but says that it reproduced by
disabling *memory* onlining.

Before we online memory for a hotplugged node, all zones are !populated.
So once we online memory for a !populated zone in online_pages(), we
trigger setup_zone_pageset().


The confusing part is that this patch checks for 3. but says it can be
reproduced by not onlining *memory*. There seems to be something missing.

Do we maybe need a proper populated_zone() check before accessing zone data?

-- 
Thanks,

David / dhildenb