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

[Uladzislau Rezki <urezki@gmail.com>]

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.
> 
> To address this, we introduced a debugfs interface that allows selectively
> enabling or disabling the vmalloc check based on the use case, optimizing
> performance.
> 
> By default, vmalloc check for usercopy is enabled.
> 
> To disable the vmalloc check:
>         echo Y > /sys/kernel/debug/bypass_usercopy_vmalloc_check
> 
> After executing the above command, the XDP performance returns to 25
> Gbits/sec.
> 
To what Matthew has asked, could you please also specify the kernel version
you run your experiment on? Apart of that please describe your system and HW.

Thank you!

--
Uladzislau Rezki