Re: [PATCH 1/1] mm/mm_init: compute hash mask with unsigned arithmetic

[Matthew Wilcox]

On Sun, Aug 09, 2026 at 06:14:00PM -0700, Andrew Morton wrote:
> > >> Use an unsigned literal so the mask is computed with unsigned
> > >> arithmetic. Supported compiler settings already produce the same
> > >> result, so this is a source-level cleanup with no functional change.
> > > If complier already takes care of this then why do we want this patch?
> >   
> > Fair point. Since supported compilers already produce the intended 
> > result and this patch has no functional impact, the benefit is only 
> > making the unsigned arithmetic explicit. So please disregard this patch.
> 
> It's a very small thing, but I believe the patch improves the code.
> 
> I mean, we erroneously compute a large negative number then subtract 1
> from it then copy that larger negative number into a signed scalar.

That is an erroneous description of this code.

1 << 31 is the largest-magnitude negative number, ie it's INT_MIN.
We then subtract one from it, so it wraps back around to INT_MAX.
So the number assigned to *_hash_mask is always positive.

> The copied bit pattern happens to be what we'd have got if the code had
> been correct, but the code isn't correct!

The code is correct as long as we compile with -fwrapv or whatever that
got renamed to.  And I don't see that changing.