Re: [PATCH v2 2/3] sched_ext: Track bits[] storage size in struct scx_cmask

[Tejun Heo]

On Tue, May 19, 2026 at 12:11:35AM +0200, Andrea Righi wrote:
> > +/**
> > + * scx_cmask_reframe - Reshape @m's active range without resizing storage
> > + * @m: cmask to reframe
> > + * @base: new active range base
> > + * @nr_cids: new active range length, must fit within @m->alloc_words
> > + *
> > + * Body bits within the new range become garbage - only the head and tail
> > + * words are zeroed to keep the padding invariant.
> > + */
> > +static inline void scx_cmask_reframe(struct scx_cmask *m, u32 base, u32 nr_cids)
> > +{
> > +	if (WARN_ON_ONCE(SCX_CMASK_NR_WORDS(nr_cids) > m->alloc_words))
> > +		return;
> 
> Considering that:
> 
>  #define SCX_CMASK_NR_WORDS(nr_cids)    (((nr_cids) + 63) / 64 + 1)
> 
> If we pass nr_cids == UINT_MAX here, we have:
> 
>  CMASK_NR_WORDS(UINT_MAX) = (UINT_MAX + 63)/64 + 1 = 62/64 + 1 = 1 (wraps)
> 
> Should we simply reject if it's greater than a certain reasonable upper bound?

I'm not sure what we do matters. No matter what, this would be a clear bug
and an unlikely one at that. As long as the backtrace is dumped, I think
anything is fine.

Thanks.

-- 
tejun