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

["Björn Töpel" <bjorn@kernel.org>]

Hou Tao <houtao@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@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()?
>
> 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.


Björn