Re: [PATCH v1 1/2] uaccess: Add __user_write_access_begin().

On Thu, Oct 23, 2025 at 1:29 AM David Laight
[off-list ref] wrote:

On Wed, 22 Oct 2025 19:37:27 -1000
Linus Torvalds [off-list ref] wrote:

quoted

On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima [off-list ref] wrote:

quoted

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.

IIUC, scoped_user_write_access() is simply inlined to
masked_user_access_begin() or user_access_begin(), and this
is the case where I saw no improvement or even worse performance.

        David

quoted

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.

Yes, it's only 3 instructions on x86_64, but by saving them
I saw better performance constantly.  Please see the numbers here.
https://lore.kernel.org/lkml/20251024051653.66329-1-kuniyu@google.com/ (local)

quoted

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