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Date:   Tue, 9 Nov 2021 08:03:00 +0100
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To:     Eric Dumazet <eric.dumazet@gmail.com>,
        Alexey Makhalov <amakhalov@vmware.com>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Oscar Salvador <osalvador@suse.de>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <908909e0-4815-b580-7ff5-d824d36a141c@redhat.com>
 <20211108202325.20304-1-amakhalov@vmware.com>
 <2e191db3-286f-90c6-bf96-3f89891e9926@gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v3] mm: fix panic in __alloc_pages
In-Reply-To: <2e191db3-286f-90c6-bf96-3f89891e9926@gmail.com>
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On 09.11.21 03:08, Eric Dumazet wrote:
> 
> 
> On 11/8/21 12:23 PM, Alexey Makhalov wrote:
>> There is a kernel panic caused by pcpu_alloc_pages() passing
>> offlined and uninitialized node to alloc_pages_node() leading
>> to panic by NULL dereferencing uninitialized NODE_DATA(nid).
>>
>>  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
>>  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
>>
>> Panic can be easily reproduced by disabling udev rule for
>> automatic onlining hot added CPU followed by CPU with
>> memoryless node (NUMA node with CPU only) hot add.
>>
>> Hot adding CPU and memoryless node does not bring the node
>> to online state. Memoryless node will be onlined only during
>> the onlining its CPU.
>>
>> 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)
>>
>> Percpu code is doing allocations for all possible CPUs. The
>> issue happens when it serves hot added but not yet onlined
>> CPU when its node is in 2nd state. This node is not ready
>> to use, fallback to numa_mem_id().
>>
>> Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
>> Reviewed-by: David Hildenbrand <david@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Dennis Zhou <dennis@kernel.org>
>> Cc: Tejun Heo <tj@kernel.org>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: stable@vger.kernel.org
>> ---
>>  mm/percpu-vm.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
>> index 2054c9213..f58d73c92 100644
>> --- a/mm/percpu-vm.c
>> +++ b/mm/percpu-vm.c
>> @@ -84,15 +84,19 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
>>  			    gfp_t gfp)
>>  {
>>  	unsigned int cpu, tcpu;
>> -	int i;
>> +	int i, nid;
>>  
>>  	gfp |= __GFP_HIGHMEM;
>>  
>>  	for_each_possible_cpu(cpu) {
>> +		nid = cpu_to_node(cpu);
>> +		if (nid == NUMA_NO_NODE || !node_online(nid))
>> +			nid = numa_mem_id();
> 
> Maybe we should fail this fallback if (gfp & __GFP_THISNODE) ?

... and what to do then? Fail the allocation? We could do that, but ...

> 
> Or maybe there is no support for this constraint in per-cpu allocator anyway.
> 

... looking at mm/percpu.c, I don't think there are any users (IOW not
supported?).

> I am a bit worried that we do not really know if pages are
> allocated on the right node or not.

Even without __GFP_THISNODE it's sub-optimal. But if there is no memory
on that node, there is barely anything we can do than falling back.

-- 
Thanks,

David / dhildenb