Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().

Dave Hansen
2025-10-24T14:05:52Z

On 10/23/25 22:16, Kuniyuki Iwashima wrote:
>> This makes me nervous. The access_ok() check is quite a distance away.
>> I'd kinda want to see some performance numbers before doing this. Is
>> removing a single access_ok() even measurable?
> I noticed I made a typo in commit message, s/tcp_rr/udp_rr/.
>
> epoll_put_uevent() can be called multiple times in a single
> epoll_wait(), and we can see 1.7% more pps on UDP even when
> 1 thread has 1000 sockets only:
>
> server: $ udp_rr --nolog -6 -F 1000 -T 1 -l 3600
> client: $ udp_rr --nolog -6 -F 1000 -T 256 -l 3600 -c -H $SERVER
> server: $ nstat > /dev/null; sleep 10; nstat | grep -i udp
>
> Without patch (2 stac/clac):
> Udp6InDatagrams 2205209 0.0
>
> With patch (1 stac/clac):
> Udp6InDatagrams 2242602 0.0

I'm totally with you about removing a stac/clac:

https://lore.kernel.org/lkml/20250228203722.CAEB63AC@davehans-spike.ostc.intel.com/

The thing I'm worried about is having the access_ok() so distant
from the unsafe_put_user(). I'm wondering if this:

- __user_write_access_begin(uevent, sizeof(*uevent));
+ if (!user_write_access_begin(uevent, sizeof(*uevent))
+ return NULL;
unsafe_put_user(revents, &uevent->events, efault);
unsafe_put_user(data, &uevent->data, efault);
user_access_end();

is measurably slower than what was in your series. If it is
not measurably slower, then the series gets simpler because it
does not need to refactor user_write_access_begin(). It also ends
up more obviously correct because the access check is closer to
the unsafe_put_user() calls.

Also, the extra access_ok() is *much* cheaper than stac/clac.