WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342

Tue Aug 29 05:54:28 PDT 2023

Björn Töpel <bjorn at kernel.org> writes:

> Hou Tao <houtao at huaweicloud.com> writes:
>
>> Hi,
>>
>> On 8/27/2023 10:53 PM, Yonghong Song wrote:
>>>
>>>
>>> On 8/27/23 1:37 AM, Björn Töpel wrote:
>>>> Björn Töpel <bjorn at kernel.org> writes:
>>>>
>>>>> Hou Tao <houtao at huaweicloud.com> writes:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> On 8/26/2023 5:23 PM, Björn Töpel wrote:
>>>>>>> Hou Tao <houtao at huaweicloud.com> writes:
>>>>>>>
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> On 8/25/2023 11:28 PM, Yonghong Song wrote:
>>>>>>>>>
>>>>>>>>> On 8/25/23 3:32 AM, Björn Töpel wrote:
>>>>>>>>>> I'm chasing a workqueue hang on RISC-V/qemu (TCG), using the bpf
>>>>>>>>>> selftests on bpf-next 9e3b47abeb8f.
>>>>>>>>>>
>>>>>>>>>> I'm able to reproduce the hang by multiple runs of:
>>>>>>>>>>    | ./test_progs -a link_api -a linked_list
>>>>>>>>>> I'm currently investigating that.
>>>>>>>>>>
>>>>>>>>>> But! Sometimes (every blue moon) I get a warn_on_once hit:
>>>>>>>>>>    | ------------[ cut here ]------------
>>>>>>>>>>    | WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342
>>>>>>>>>> bpf_mem_refill+0x1fc/0x206
>>>>>>>>>>    | Modules linked in: bpf_testmod(OE)
>>>>>>>>>>    | CPU: 3 PID: 261 Comm: test_progs-cpuv Tainted: G           OE
>>>>>>>>>> N 6.5.0-rc5-01743-gdcb152bb8328 #2
>>>>>>>>>>    | Hardware name: riscv-virtio,qemu (DT)
>>>>>>>>>>    | epc : bpf_mem_refill+0x1fc/0x206
>>>>>>>>>>    |  ra : irq_work_single+0x68/0x70
>>>>>>>>>>    | epc : ffffffff801b1bc4 ra : ffffffff8015fe84 sp :
>>>>>>>>>> ff2000000001be20
>>>>>>>>>>    |  gp : ffffffff82d26138 tp : ff6000008477a800 t0 :
>>>>>>>>>> 0000000000046600
>>>>>>>>>>    |  t1 : ffffffff812b6ddc t2 : 0000000000000000 s0 :
>>>>>>>>>> ff2000000001be70
>>>>>>>>>>    |  s1 : ff5ffffffffe8998 a0 : ff5ffffffffe8998 a1 :
>>>>>>>>>> ff600003fef4b000
>>>>>>>>>>    |  a2 : 000000000000003f a3 : ffffffff80008250 a4 :
>>>>>>>>>> 0000000000000060
>>>>>>>>>>    |  a5 : 0000000000000080 a6 : 0000000000000000 a7 :
>>>>>>>>>> 0000000000735049
>>>>>>>>>>    |  s2 : ff5ffffffffe8998 s3 : 0000000000000022 s4 :
>>>>>>>>>> 0000000000001000
>>>>>>>>>>    |  s5 : 0000000000000007 s6 : ff5ffffffffe8570 s7 :
>>>>>>>>>> ffffffff82d6bd30
>>>>>>>>>>    |  s8 : 000000000000003f s9 : ffffffff82d2c5e8 s10:
>>>>>>>>>> 000000000000ffff
>>>>>>>>>>    |  s11: ffffffff82d2c5d8 t3 : ffffffff81ea8f28 t4 :
>>>>>>>>>> 0000000000000000
>>>>>>>>>>    |  t5 : ff6000008fd28278 t6 : 0000000000040000
>>>>>>>>>>    | status: 0000000200000100 badaddr: 0000000000000000 cause:
>>>>>>>>>> 0000000000000003
>>>>>>>>>>    | [<ffffffff801b1bc4>] bpf_mem_refill+0x1fc/0x206
>>>>>>>>>>    | [<ffffffff8015fe84>] irq_work_single+0x68/0x70
>>>>>>>>>>    | [<ffffffff8015feb4>] irq_work_run_list+0x28/0x36
>>>>>>>>>>    | [<ffffffff8015fefa>] irq_work_run+0x38/0x66
>>>>>>>>>>    | [<ffffffff8000828a>] handle_IPI+0x3a/0xb4
>>>>>>>>>>    | [<ffffffff800a5c3a>] handle_percpu_devid_irq+0xa4/0x1f8
>>>>>>>>>>    | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
>>>>>>>>>>    | [<ffffffff800ae570>] ipi_mux_process+0xac/0xfa
>>>>>>>>>>    | [<ffffffff8000a8ea>] sbi_ipi_handle+0x2e/0x88
>>>>>>>>>>    | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
>>>>>>>>>>    | [<ffffffff807ee70e>] riscv_intc_irq+0x36/0x4e
>>>>>>>>>>    | [<ffffffff812b5d3a>] handle_riscv_irq+0x54/0x86
>>>>>>>>>>    | [<ffffffff812b6904>] do_irq+0x66/0x98
>>>>>>>>>>    | ---[ end trace 0000000000000000 ]---
>>>>>>>>>>
>>>>>>>>>> Code:
>>>>>>>>>>    | static void free_bulk(struct bpf_mem_cache *c)
>>>>>>>>>>    | {
>>>>>>>>>>    |     struct bpf_mem_cache *tgt = c->tgt;
>>>>>>>>>>    |     struct llist_node *llnode, *t;
>>>>>>>>>>    |     unsigned long flags;
>>>>>>>>>>    |     int cnt;
>>>>>>>>>>    |
>>>>>>>>>>    |     WARN_ON_ONCE(tgt->unit_size != c->unit_size);
>>>>>>>>>>    | ...
>>>>>>>>>>
>>>>>>>>>> I'm not well versed in the memory allocator; Before I dive into
>>>>>>>>>> it --
>>>>>>>>>> has anyone else hit it? Ideas on why the warn_on_once is hit?
>>>>>>>>> Maybe take a look at the patch
>>>>>>>>>    822fb26bdb55  bpf: Add a hint to allocated objects.
>>>>>>>>>
>>>>>>>>> In the above patch, we have
>>>>>>>>>
>>>>>>>>> +       /*
>>>>>>>>> +        * Remember bpf_mem_cache that allocated this object.
>>>>>>>>> +        * The hint is not accurate.
>>>>>>>>> +        */
>>>>>>>>> +       c->tgt = *(struct bpf_mem_cache **)llnode;
>>>>>>>>>
>>>>>>>>> I suspect that the warning may be related to the above.
>>>>>>>>> I tried the above ./test_progs command line (running multiple
>>>>>>>>> at the same time) and didn't trigger the issue.
>>>>>>>> The extra 8-bytes before the freed pointer is used to save the
>>>>>>>> pointer
>>>>>>>> of the original bpf memory allocator where the freed pointer came
>>>>>>>> from,
>>>>>>>> so unit_free() could free the pointer back to the original
>>>>>>>> allocator to
>>>>>>>> prevent alloc-and-free unbalance.
>>>>>>>>
>>>>>>>> I suspect that a wrong pointer was passed to bpf_obj_drop, but do
>>>>>>>> not
>>>>>>>> find anything suspicious after checking linked_list. Another
>>>>>>>> possibility
>>>>>>>> is that there is write-after-free problem which corrupts the extra
>>>>>>>> 8-bytes before the freed pointer. Could you please apply the
>>>>>>>> following
>>>>>>>> debug patch to check whether or not the extra 8-bytes are
>>>>>>>> corrupted ?
>>>>>>> Thanks for getting back!
>>>>>>>
>>>>>>> I took your patch for a run, and there's a hit:
>>>>>>>    | bad cache ff5ffffffffe8570: got size 96 work
>>>>>>> ffffffff801b19c8, cache ff5ffffffffe8980 exp size 128 work
>>>>>>> ffffffff801b19c8
>>>>>>
>>>>>> The extra 8-bytes are not corrupted. Both of these two
>>>>>> bpf_mem_cache are
>>>>>> valid and there are in the cache array defined in bpf_mem_caches. BPF
>>>>>> memory allocator allocated the pointer from 96-bytes sized-cache,
>>>>>> but it
>>>>>> tried to free the pointer through 128-bytes sized-cache.
>>>>>>
>>>>>> Now I suspect there is no 96-bytes slab in your system and ksize(ptr -
>>>>>> LLIST_NODE_SZ) returns 128, so bpf memory allocator selected the
>>>>>> 128-byte sized-cache instead of 96-bytes sized-cache. Could you please
>>>>>> check the value of KMALLOC_MIN_SIZE in your kernel .config and
>>>>>> using the
>>>>>> following command to check whether there is 96-bytes slab in your
>>>>>> system:
>>>>>
>>>>> KMALLOC_MIN_SIZE is 64.
>>>>>
>>>>>> $ cat /proc/slabinfo |grep kmalloc-96
>>>>>> dma-kmalloc-96         0      0     96   42    1 : tunables    0    0
>>>>>> 0 : slabdata      0      0      0
>>>>>> kmalloc-96          1865   2268     96   42    1 : tunables    0    0
>>>>>> 0 : slabdata     54     54      0
>>>>>>
>>>>>> In my system, slab has 96-bytes cached, so grep outputs something,
>>>>>> but I
>>>>>> think there will no output in your system.
>>>>>
>>>>> You're right! No kmalloc-96.
>>>>
>>>> To get rid of the warning, limit available sizes from
>>>> bpf_mem_alloc_init()?
>>
>> It is not enough. We need to adjust size_index accordingly during
>> initialization. Could you please try the attached patch below ? It is
>> not a formal patch and I am considering to disable prefilling for these
>> redirected bpf_mem_caches.
>
> Sorry for the slow response; I'll take it for a spin today.

The warning is indeed gone with this fix.

Björn