Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/12/3 21:51, Uladzislau Rezki wrote:
> On Tue, Dec 03, 2024 at 09:45:09PM +0800, Kefeng Wang wrote:
>>
>>
>> On 2024/12/3 21:39, Uladzislau Rezki wrote:
>>> On Tue, Dec 03, 2024 at 09:30:09PM +0800, Kefeng Wang wrote:
>>>>
>>>>
>>>> On 2024/12/3 21:10, zuoze wrote:
>>>>>
>>>>>
>>>>> 在 2024/12/3 20:39, Uladzislau Rezki 写道:
>>>>>> On Tue, Dec 03, 2024 at 07:23:44PM +0800, zuoze wrote:
>>>>>>> We have implemented host-guest communication based on the TUN device
>>>>>>> using XSK[1]. The hardware is a Kunpeng 920 machine (ARM architecture),
>>>>>>> and the operating system is based on the 6.6 LTS version with kernel
>>>>>>> version 6.6. The specific stack for hotspot collection is as follows:
>>>>>>>
>>>>>>> -  100.00%     0.00%  vhost-12384  [unknown]      [k] 0000000000000000
>>>>>>>       - ret_from_fork
>>>>>>>          - 99.99% vhost_task_fn
>>>>>>>             - 99.98% 0xffffdc59f619876c
>>>>>>>                - 98.99% handle_rx_kick
>>>>>>>                   - 98.94% handle_rx
>>>>>>>                      - 94.92% tun_recvmsg
>>>>>>>                         - 94.76% tun_do_read
>>>>>>>                            - 94.62% tun_put_user_xdp_zc
>>>>>>>                               - 63.53% __check_object_size
>>>>>>>                                  - 63.49% __check_object_size.part.0
>>>>>>>                                       find_vmap_area
>>>>>>>                               - 30.02% _copy_to_iter
>>>>>>>                                    __arch_copy_to_user
>>>>>>>                      - 2.27% get_rx_bufs
>>>>>>>                         - 2.12% vhost_get_vq_desc
>>>>>>>                              1.49% __arch_copy_from_user
>>>>>>>                      - 0.89% peek_head_len
>>>>>>>                           0.54% xsk_tx_peek_desc
>>>>>>>                      - 0.68% vhost_add_used_and_signal_n
>>>>>>>                         - 0.53% eventfd_signal
>>>>>>>                              eventfd_signal_mask
>>>>>>>                - 0.94% handle_tx_kick
>>>>>>>                   - 0.94% handle_tx
>>>>>>>                      - handle_tx_copy
>>>>>>>                         - 0.59% vhost_tx_batch.constprop.0
>>>>>>>                              0.52% tun_sendmsg
>>>>>>>
>>>>>>> It can be observed that most of the overhead is concentrated in the
>>>>>>> find_vmap_area function.
>>>>>>>
...
>>
> Thank you. Then you have tons of copy_to_iter/copy_from_iter calls
> during your test case. Per each you need to find an area which might
> be really heavy.

Exactly, no vmalloc check before 0aef499f3172 （"mm/usercopy: Detect 
vmalloc overruns"), so no burden in find_vmap_area in old kernel.

> 
> How many CPUs in a system you have?
> 

128 core