On Thu, Oct 23, 2025 at 1:29 AM David Laight <[email protected]> wrote: > > On Wed, 22 Oct 2025 19:37:27 -1000 > Linus Torvalds <[email protected]> wrote: > > > On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <[email protected]> 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.
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 > > > > > 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/[email protected]/ > > > > 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 > > >
