Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

[Linus Torvalds]

On Fri, Mar 3, 2023 at 9:39 AM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> So the key is: memset is underperforming at least on x86-64 for
> certain sizes and the init-on-alloc thingy makes it used significantly
> more, exacerbating the problem

One reason that the kernel memset isn't as optimized as memcpy, is
simply because under normal circumstances it shouldn't be *used* that
much outside of page clearing and constant-sized structure
initialization.

Page clearing is fine, and constant-sized structure inits are also
generally fine (ie the compiler does the small ones directly).

So this is literally a problem with pointless automated memset,
introduced by that hardening option. And hardening people generally
simply don't care about performance, and the people who _do _care
about performance usually don't enable the known-expensive crazy
stuff.

Honestly, I think the INIT_ONCE stuff is actively detrimental, and
only hides issues (and in this case adds its own). So I can't but help
to say "don't do that then". I think it's literally stupid to clear
allocations by default.

I'm not opposed to improving memset, but honestly, if the argument is
based on the stupid hardening behavior, I really think *that* needs to
be fixed first.

               Linus