Re: [PATCH v3] eventpoll: Convert epoll_put_uevent() to scoped user access

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 07/03/2026 à 21:07, Eric Dumazet a écrit :
> Saves two function calls, and one stac/clac pair.
> 
> stac/clac is rather expensive on older cpus like Zen 2.
> 
> A synthetic network stress test gives a ~1.5% increase of pps
> on AMD Zen 2.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Kuniyuki Iwashima <kuniyu@google.com>
> ---
> Prior attempts by Kuniyuki, in case some background is needed:
> 
> v2 : https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org%2Flkml%2F2025%2F10%2F28%2F1553&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C85a6664e663a4b224ce208de7c852734%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639085108483835748%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=tYX5SAFAkM%2B4b2ooJnd8A%2FCO3hp8Evn1AmB0R5TFI4M%3D&reserved=0
> v1:  https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20251023000535.2897002-1-kuniyu%40google.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C85a6664e663a4b224ce208de7c852734%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639085108483859728%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=wfPZz01ZLu%2B4HABR941GaOE6PnsLlsAnm%2F0xVEsjM8g%3D&reserved=0
> 
> Thanks !
> ---
>   include/linux/eventpoll.h | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
> index ccb478eb174bc2e4454cee59f805f10cfd5bb5fb..ea9ca0e4172aab13b8e576258e1d457a913dcebf 100644
> --- a/include/linux/eventpoll.h
> +++ b/include/linux/eventpoll.h
> @@ -82,11 +82,14 @@ static inline struct epoll_event __user *
>   epoll_put_uevent(__poll_t revents, __u64 data,
>   		 struct epoll_event __user *uevent)
>   {
> -	if (__put_user(revents, &uevent->events) ||
> -	    __put_user(data, &uevent->data))
> -		return NULL;
> -
> +	scoped_user_write_access_size(uevent, sizeof(*uevent), efault) {

You can use scoped_user_write_access() instead which is defined as

#define scoped_user_write_access(udst, elbl)				\
	scoped_user_write_access_size(udst, sizeof(*(udst)), elbl)

> +		unsafe_put_user(revents, &uevent->events, efault);
> +		unsafe_put_user(data, &uevent->data, efault);
> +	}
>   	return uevent+1;
> +
> +efault:
> +	return NULL;
>   }
>   #endif
>   
> 
> base-commit: 4ae12d8bd9a830799db335ee661d6cbc6597f838
> prerequisite-patch-id: f6002c357582927a383603a22e69bc0d7a5b9528