Re: Serious regression on 6.1.x-stable caused by "bounds: support non-power-of-two CONFIG_NR_CPUS"

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 29, 2024 at 07:07:39AM +0300, Михаил Новоселов wrote:
> >> It was backported to 6.1.84, 6.1.84 has problems, 6/1/83 does not, the newest
> >> 6.1.88 still has this problem.
> > 
> > Does v6.8.3 (which contains cf778fff03be) have this problem?
> > How about current Linus master?
> 
> 6.1.88 - has problem
> 6.6.27 - does not have problem
> 6.9-rc from commit efdfbbc4dcc8f98754056971f88af0f7ff906144 https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git - does not have problem
> 
> 6.8.3 was not tested, but we can test it if needed.

How curious.

> > What kernel config were you using?  I don't see that info on
> > https://linux-hardware.org/?probe=9c92ac1222
> > (maybe my tired eyes can't see it)
> 
> Kernel config for 6.1: https://abf.io/import/kernel-6.1/blob/bcb3e9611f/kernel-x86_64.config

CONFIG_NR_CPUS=8192

> For 6.6: https://abf.io/import/kernel-6.6/blob/7404a4d3d5/kernel-x86_64.config

CONFIG_NR_CPUS=8192

Since you're using a power-of-two, this should have been a no-op.
But bits_per() doesn't work the way I thought it did!

#define bits_per(n)                             \
(                                               \
        __builtin_constant_p(n) ? (             \
                ((n) == 0 || (n) == 1)          \
                        ? 1 : ilog2(n) + 1      \
        ) :                                     \

CONFIG_NR_CPUS is obviously a constant, and larger than 1, so we end up
calling ilog2(n) + 1.  So we allocate one extra bit.

I should have changed this to
DEFINE(NR_CPUS_BITS, bits_per(CONFIG_NR_CPUS - 1))

Can you test that and report back?  I'll prepare a fix for mainline in
the meantime.