On Tue, Aug 05, 2025 at 09:42:14AM -0700, Dave Hansen wrote:
> On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> >
> > Add largest_zero_folio() routine so that huge_zero_folio can be
> > used directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> > return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> > if we failed to allocate a huge_zero_folio.
>
> This changelog is telling a lot of the "what" but none of the "why".
>
> The existing huge zero folio API is for users that have an mm. This is
> *only* for folks that want a huge zero folio but don't have an mm.
> That's _why_ this function is needed. It's in this series because there
> was no way to get a huge zero folio before the permanent one was
> introduced in this series.
>
> Can we please get some of that into the function comment and changelog?
> It's critical.
Valid point. I will include that in the next version.
>
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 78ebceb61d0e..c44a6736704b 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
> > return split_folio_to_list_to_order(folio, NULL, new_order);
> > }
> >
> > +/*
> > + * largest_zero_folio - Get the largest zero size folio available
> > + *
> > + * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
> > + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> > + *
> > + * Deduce the size of the folio with folio_size instead of assuming the
> > + * folio size.
> > + */
>
> This comment needs to get fleshed out. It at _least_ needs to say
> something along the lines of:
>
> Use this when a huge zero folio is needed but there is no mm
> lifetime to tie it to. Basically, use this when you can't use
> mm_get_huge_zero_folio().
>
Agreed.
> > +static inline struct folio *largest_zero_folio(void)
> > +{
> > + struct folio *folio = get_static_huge_zero_folio();
> > +
> > + if (folio)
> > + return folio;
>
> There needs to be a newline in here.
>
> > + return page_folio(ZERO_PAGE(0));
> > +}
> > #endif /* _LINUX_HUGE_MM_H */
>
> The code is fine, but the changelog and comments need quite a bit of work.
Sounds good to me. I will make the changes. Thanks Dave.
--
Pankaj
