Re: [PATCH] mm: fix panic in __alloc_pages

Alexey Makhalov
2021-11-02T08:48:34Z

﻿On 11/2/21, 1:12 AM, "David Hildenbrand" <david@redhat.com> wrote:

Thanks for reviews,

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.
This approach behaves in the same way as CPU was not yet added. (state #1).
So, we can think of state #2 as state #1 when CPU is not present.

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().
You are correct. In case of memory hot add, it is true. But in case of adding
CPU with memoryless node, try_node_online() will be called only during CPU
onlining, see cpu_up().

Is there any reason why try_online_node() resides in cpu_up() and not in add_cpu()?
I think it would be correct to online node during the CPU hot add to align with
memory hot add.

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.
This is the hot adding of both new CPU and new _memoryless_ node (with CPU only)
And onlining CPU makes its node online. Disabling CPU onlining puts new node
into state #2, which leads to repro.

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,
--Alexey