linux-kernel - Re: [PATCH 4/5] mm: add largest_zero

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Message-ID: <ib67jxoli3qaw4ntmgrhhumlkrn4hicjnjzbocmwwi7t2vls73@fczc5msyhy37>
Date: Wed, 6 Aug 2025 09:59:15 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, 
	Ryan Roberts <ryan.roberts@....com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>, 
	Mike Rapoport <rppt@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Michal Hocko <mhocko@...e.com>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....com>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, willy@...radead.org, x86@...nel.org, linux-block@...r.kernel.org, 
	Ritesh Harjani <ritesh.list@...il.com>, linux-fsdevel@...r.kernel.org, 
	"Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org, gost.dev@...sung.com, hch@....de, 
	Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH 4/5] mm: add largest_zero_folio() routine

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@...sung.com>
> > 
> > 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