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

[zuoze <zuoze1@huawei.com>]

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.

[1]: https://www.kernel.org/doc/html/latest/networking/af_xdp.html

在 2024/12/3 12:11, Matthew Wilcox 写道:
> On Tue, Dec 03, 2024 at 10:31:59AM +0800, Ze Zuo wrote:
>> The commit 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns") introduced
>> vmalloc check for usercopy. However, in subsystems like networking, when
>> memory allocated using vmalloc or vmap is subsequently copied using
>> functions like copy_to_iter/copy_from_iter, the check is triggered. This
>> adds overhead in the copy path, such as the cost of searching the
>> red-black tree, which increases the performance burden.
>>
>> We found that after merging this patch, network bandwidth performance in
>> the XDP scenario significantly dropped from 25 Gbits/sec to 8 Gbits/sec,
>> the hardened_usercopy is enabled by default.
> 
> What is "the XDP scenario", exactly?  Are these large or small packets?
> What's taking the time in find_vmap_area()?  Is it lock contention?
>