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

[Linus Torvalds]

On Sun, Mar 5, 2023 at 10:17 AM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> There are a few main issues with MAXSMP:

It's probably worth noting that most architectures don't even support
MAXSMP at all.

Only x86-64 does.

For example, ia64 and sparc64, which both did techncially support a
lot of cores, just made "cpumask_t" huge, and had no support for the
whole "use a pointer to an indirect allocation".

That ends up meaning that you allocate those huge structures on the
stack or just make other structures enormous when they contain a CPU
mask, but it mostly works. It's a horrid, horrid model, though. But at
least ia64 had 64kB stacks anyway, and in the book of "bad engineering
decisions of Itanium", this is all just a footnote.

arm64 also has that "range 2 4096" for number of CPUs but defaults to
a much saner 256 cpus.

I suspect (and sincerely hope) that nobody actually tries to use an
arm64 build with that 4k cpu build. If/when arm64 actually does get up
to that 'thousands of cores" situation, they'll hopefully enable the
MAXSMP kind of indirection and off-stack cpu mask arrays.

So MAXSMP and the whole CPUMASK_OFFSTACK option is an architecture
choice, and you don't have to do it the way x86-64 does it. But the
x86 choice is likely the best tested and thought out by far.

For example, POWERPC technically supports CPUMASK_OFFSTACK too, but
really only in theory. On powerpc, you have

    config NR_CPUS
          range 2 8192 if SMP
          default "32" if PPC64

so while configuration the range is technically up to 8k CPUs, I doubt
people use that value very much. And we have

        select CPUMASK_OFFSTACK if NR_CPUS >= 8192

so it only uses that OFFSTACK one if you pick exactly 8192 CPUs (which
presumably nobody does in real life outside of build testing - it's
not the default, and I think most of the POWER range tops up in the
192 core range, eg E980 with 16 sockets of 12 cores each).

So I suspect that x86-64 is the *only* one to actually use this
widely, and I think distros have been *much* too eager to do so.

The fact that most distros default to

    CONFIG_MAXSMP=y
    CONFIG_NR_CPUS=8192

seems pretty crazy, when I have a hard time finding anything with more
than 192 cores.  I'm sure they exist. But do they _really_ run
unmodified vendor kernels?

               Linus