Re: [PATCH 4/5] mm: add mm_get_static_huge_zero_folio() routine

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Thu, Jun 12, 2025 at 07:09:34AM -0700, Dave Hansen wrote:
> On 6/12/25 03:50, Pankaj Raghav wrote:
> > +/*
> > + * mm_get_static_huge_zero_folio - Get a PMD sized zero folio
> 
> Isn't that a rather inaccurate function name and comment?
I agree. I also felt it was not a good name for the function.

> 
> The third line of the function literally returns a non-PMD-sized zero folio.
> 
> > + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
> > + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> > + *
> > + * Deduce the size of the folio with folio_size instead of assuming the
> > + * folio size.
> > + */
> > +static inline struct folio *mm_get_static_huge_zero_folio(void)
> > +{
> > +	if(IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
> > +		return READ_ONCE(huge_zero_folio);
> > +	return page_folio(ZERO_PAGE(0));
> > +}
> 
> This doesn't tell us very much about when I should use:
> 
> 	mm_get_static_huge_zero_folio()
> vs.
> 	mm_get_huge_zero_folio(mm)
> vs.
> 	page_folio(ZERO_PAGE(0))
> 
> What's with the "mm_" in the name? Usually "mm" means "mm_struct" not
> Memory Management. It's really weird to prefix something that doesn't
> take an "mm_struct" with "mm_"

Got it. Actually, I was not aware of this one.

> 
> Isn't the "get_" also a bad idea since mm_get_huge_zero_folio() does its
> own refcounting but this interface does not?
> 

Agree.

> Shouldn't this be something more along the lines of:
> 
> /*
>  * pick_zero_folio() - Pick and return the largest available zero folio
>  *
>  * mm_get_huge_zero_folio() is preferred over this function. It is more
>  * flexible and can provide a larger zero page under wider
>  * circumstances.
>  *
>  * Only use this when there is no mm available.
>  *
>  * ... then other comments
>  */
> static inline struct folio *pick_zero_folio(void)
> {
> 	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
> 		return READ_ONCE(huge_zero_folio);
> 	return page_folio(ZERO_PAGE(0));
> }
> 
> Or, maybe even name it _just_: zero_folio()

I think zero_folio() sounds like a good and straightforward name. In
most cases it will return a ZERO_PAGE() folio. If
CONFIG_STATIC_PMD_ZERO_PAGE is enabled, then we return a PMD page.

Thanks for all your comments Dave.

--
Pankaj