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From: Kuniyuki Iwashima To: dave.hansen@intel.com Cc: alex@ghiti.fr, aou@eecs.berkeley.edu, axboe@kernel.dk, bp@alien8.de, brauner@kernel.org, catalin.marinas@arm.com, christophe.leroy@csgroup.eu, dave.hansen@linux.intel.com, edumazet@google.com, hpa@zytor.com, kuni1840@gmail.com, kuniyu@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, maddy@linux.ibm.com, mingo@redhat.com, mpe@ellerman.id.au, npiggin@gmail.com, palmer@dabbelt.com, pjw@kernel.org, tglx@linutronix.de, torvalds@linux-foundation.org, will@kernel.org, x86@kernel.org Content-Type: text/plain; charset="UTF-8" From: Dave Hansen Date: Thu, 23 Oct 2025 12:40:59 -0700 > On 10/22/25 17:04, Kuniyuki Iwashima wrote: > > --- 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; > > - > > - return uevent+1; > > + __user_write_access_begin(uevent, sizeof(*uevent)); > > + unsafe_put_user(revents, &uevent->events, efault); > > + unsafe_put_user(data, &uevent->data, efault); > > + user_access_end(); > > + return uevent + 1; > > +efault: > > + user_access_end(); > > + return NULL; > > } > > #endif > > 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 >>> 2242602 / 2205209 * 100 101.6956669413194 I also took a microbenchmark with bpftrace and we can see more invocations of ep_try_send_events_ns() finish faster, and 4% more total calls: $ sudo bpftrace -e ' k:ep_try_send_events { @start[cpu] = nsecs; } kr:ep_try_send_events { if (@start[cpu]) { $delay = nsecs - @start[cpu]; delete(@start[cpu]); @ep_try_send_events_ns = hist($delay); } } END { clear(@start); }' -c 'sleep 10' Without patch: @ep_try_send_events_ns: [256, 512) 2483257 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [512, 1K) 850735 |@@@@@@@@@@@@@@@@@ | [1K, 2K) 254027 |@@@@@ | [2K, 4K) 26646 | | [4K, 8K) 1358 | | [8K, 16K) 66 | | [16K, 32K) 3 | | With patch: @ep_try_send_events_ns: [256, 512) 2844733 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [512, 1K) 733956 |@@@@@@@@@@@@@ | [1K, 2K) 166349 |@@@ | [2K, 4K) 13495 | | [4K, 8K) 526 | | [8K, 16K) 63 | | [16K, 32K) 5 | | >>> (2844733 + 733956 + 166349 + 13495 + 526 + 63 + 5) / \ ... (2483257 + 850735 + 254027 + 26646 + 1358 + 66 + 3) * 100 103.95551329999347 > > Also, even if we go do this, shouldn't __user_write_access_begin() be > called something more like unsafe_user_write_access_begin()? Sounds good.