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

[Yonghong Song <yonghong.song@linux.dev>]

On 8/27/23 1:37 AM, Björn Töpel wrote:
> Björn Töpel <bjorn@kernel.org> writes:
> 
>> Hou Tao <houtao@huaweicloud.com> writes:
>>
>>> Hi,
>>>
>>> On 8/26/2023 5:23 PM, Björn Töpel wrote:
>>>> Hou Tao <houtao@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()?

Do you know why your system does not have kmalloc-96?

> 
> 
> Björn

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