Re: [PATCH v3] mm: fix panic in __alloc_pages

David Hildenbrand
2021-11-09T07:03:07Z

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