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

Sun Aug 27 01:37:49 PDT 2023

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()?

Björn