Re: [PATCH 2/3] io_uring: change io_get_ext_arg() to uaccess begin + end

[Keith Busch <kbusch@kernel.org>]

On Tue, Oct 22, 2024 at 02:39:03PM -0600, Jens Axboe wrote:
> In scenarios where a high frequency of wait events are seen, the copy
> of the struct io_uring_getevents_arg is quite noticeable in the
> profiles in terms of time spent. It can be seen as up to 3.5-4.5%.
> Rewrite the copy-in logic, saving about 0.5% of the time.

I'm surprised it's faster to retrieve field by field instead of a
straight copy. But you can't argue with the results!
 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  io_uring/io_uring.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 8952453ea807..612e7d66f845 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3239,6 +3239,7 @@ static int io_validate_ext_arg(unsigned flags, const void __user *argp, size_t a
>  static int io_get_ext_arg(unsigned flags, const void __user *argp,
>  			  struct ext_arg *ext_arg)
>  {
> +	const struct io_uring_getevents_arg __user *uarg = argp;
>  	struct io_uring_getevents_arg arg;
>  
>  	/*
> @@ -3256,8 +3257,13 @@ static int io_get_ext_arg(unsigned flags, const void __user *argp,
>  	 */
>  	if (ext_arg->argsz != sizeof(arg))
>  		return -EINVAL;
> -	if (copy_from_user(&arg, argp, sizeof(arg)))
> +	if (!user_access_begin(uarg, sizeof(*uarg)))
>  		return -EFAULT;
> +	unsafe_get_user(arg.sigmask, &uarg->sigmask, uaccess_end);
> +	unsafe_get_user(arg.min_wait_usec, &uarg->min_wait_usec, uaccess_end);
> +	unsafe_get_user(arg.ts, &uarg->ts, uaccess_end);
> +	unsafe_get_user(arg.sigmask_sz, &uarg->sigmask_sz, uaccess_end);
> +	user_access_end();
>  	ext_arg->min_time = arg.min_wait_usec * NSEC_PER_USEC;
>  	ext_arg->sig = u64_to_user_ptr(arg.sigmask);
>  	ext_arg->argsz = arg.sigmask_sz;
> @@ -3267,6 +3273,9 @@ static int io_get_ext_arg(unsigned flags, const void __user *argp,
>  		ext_arg->ts_set = true;
>  	}
>  	return 0;
> +uaccess_end:
> +	user_access_end();
> +	return -EFAULT;
>  }

I was looking for the 'goto' for this label and discovered it's in the
macro. Neat.

Reviewed-by: Keith Busch <kbusch@kernel.org>