From: David Laight <david.laight.linux@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Madhavan Srinivasan <maddy@linux.ibm.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Walmsley <pjw@kernel.org>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Jens Axboe <axboe@kernel.dk>,
Christian Brauner <brauner@kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
Alexandre Ghiti <alex@ghiti.fr>, "H. Peter Anvin" <hpa@zytor.com>,
Eric Dumazet <edumazet@google.com>,
Kuniyuki Iwashima <kuni1840@gmail.com>,
x86@kernel.org, linux-arm-kernel@lists.infradead.org,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/2] uaccess: Add __user_write_access_begin().
Date: Thu, 23 Oct 2025 09:29:10 +0100 [thread overview]
Message-ID: <20251023092910.2ed9cf15@pumpkin> (raw)
In-Reply-To: <CAHk-=wjXGvUnmN5ZL3nhj_J0cbiVfeHsM9Z54A55rgHRUaVOfA@mail.gmail.com>
On Wed, 22 Oct 2025 19:37:27 -1000
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> >
> > unsafe_put_user() can be used to save a stac/clac pair, but
> > masked_user_access_begin() or user_access_begin() introduces
> > an unnecessary address masking or access_ok().
> >
> > Add a low-level helper for such a use case.
>
> I really suspect that you cannot actually measure the cost of the
> extra masking, and would be much happier if you just used a regular
> "user_access_begin()" (perhaps the "user_write_access_begin()"
> variant).
Or wait for scoped_user_write_access() to get committed and then use that.
David
>
> The masking is very cheap - literally just a couple of ALU
> instructions. And unless you can actually measure some real advantage
> of avoiding it, let's not add another helper to this area.
>
> We spent a fair amount of time undoing years of "__get_user()" and
> "__put_user()" cases that didn't actually help, and sometimes only
> made it hard to see where the actual user pointer validation was done.
>
> Linus
>
WARNING: multiple messages have this Message-ID (diff)
From: David Laight <david.laight.linux@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Madhavan Srinivasan <maddy@linux.ibm.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Walmsley <pjw@kernel.org>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Jens Axboe <axboe@kernel.dk>,
Christian Brauner <brauner@kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
Alexandre Ghiti <alex@ghiti.fr>, "H. Peter Anvin" <hpa@zytor.com>,
Eric Dumazet <edumazet@google.com>,
Kuniyuki Iwashima <kuni1840@gmail.com>,
x86@kernel.org, linux-arm-kernel@lists.infradead.org,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/2] uaccess: Add __user_write_access_begin().
Date: Thu, 23 Oct 2025 09:29:10 +0100 [thread overview]
Message-ID: <20251023092910.2ed9cf15@pumpkin> (raw)
In-Reply-To: <CAHk-=wjXGvUnmN5ZL3nhj_J0cbiVfeHsM9Z54A55rgHRUaVOfA@mail.gmail.com>
On Wed, 22 Oct 2025 19:37:27 -1000
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> >
> > unsafe_put_user() can be used to save a stac/clac pair, but
> > masked_user_access_begin() or user_access_begin() introduces
> > an unnecessary address masking or access_ok().
> >
> > Add a low-level helper for such a use case.
>
> I really suspect that you cannot actually measure the cost of the
> extra masking, and would be much happier if you just used a regular
> "user_access_begin()" (perhaps the "user_write_access_begin()"
> variant).
Or wait for scoped_user_write_access() to get committed and then use that.
David
>
> The masking is very cheap - literally just a couple of ALU
> instructions. And unless you can actually measure some real advantage
> of avoiding it, let's not add another helper to this area.
>
> We spent a fair amount of time undoing years of "__get_user()" and
> "__put_user()" cases that didn't actually help, and sometimes only
> made it hard to see where the actual user pointer validation was done.
>
> Linus
>
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next prev parent reply other threads:[~2025-10-23 8:29 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-23 0:04 [PATCH v1 0/2] epoll: Save one stac/clac pair in epoll_put_uevent() Kuniyuki Iwashima
2025-10-23 0:04 ` Kuniyuki Iwashima
2025-10-23 0:04 ` [PATCH v1 1/2] uaccess: Add __user_write_access_begin() Kuniyuki Iwashima
2025-10-23 0:04 ` Kuniyuki Iwashima
2025-10-23 5:37 ` Linus Torvalds
2025-10-23 5:37 ` Linus Torvalds
2025-10-23 8:29 ` David Laight [this message]
2025-10-23 8:29 ` David Laight
2025-10-24 5:31 ` Kuniyuki Iwashima
2025-10-24 5:31 ` Kuniyuki Iwashima
2025-10-23 0:04 ` [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent() Kuniyuki Iwashima
2025-10-23 0:04 ` Kuniyuki Iwashima
2025-10-23 19:40 ` Dave Hansen
2025-10-23 19:40 ` Dave Hansen
2025-10-24 5:16 ` Kuniyuki Iwashima
2025-10-24 5:16 ` Kuniyuki Iwashima
2025-10-24 14:05 ` Dave Hansen
2025-10-24 14:05 ` Dave Hansen
2025-10-24 14:47 ` David Laight
2025-10-24 14:47 ` David Laight
2025-10-28 5:32 ` Kuniyuki Iwashima
2025-10-28 5:32 ` Kuniyuki Iwashima
2025-10-28 9:54 ` David Laight
2025-10-28 9:54 ` David Laight
2025-10-28 16:42 ` Kuniyuki Iwashima
2025-10-28 16:42 ` Kuniyuki Iwashima
2025-10-28 16:58 ` Linus Torvalds
2025-10-28 16:58 ` Linus Torvalds
2025-10-29 1:42 ` Andrew Cooper
2025-10-29 1:42 ` Andrew Cooper
2025-10-28 22:30 ` David Laight
2025-10-28 22:30 ` David Laight
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