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

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Message-ID: <xyp7neexwts566fndmfdwivcrg4qmdu2nfv2nppcjukvpog3ib@ieuhruutubos>
Date: Mon, 16 Jun 2025 12:41:30 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dave Hansen <dave.hansen@...el.com>, 
	Pankaj Raghav <p.raghav@...sung.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Ryan Roberts <ryan.roberts@....com>, Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....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>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "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, 
	linux-fsdevel@...r.kernel.org, "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org, 
	gost.dev@...sung.com, hch@....de
Subject: Re: [PATCH 4/5] mm: add mm_get_static_huge_zero_folio() routine

On Mon, Jun 16, 2025 at 11:14:07AM +0200, David Hildenbrand wrote:
> On 12.06.25 22:54, Pankaj Raghav (Samsung) wrote:
> > 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.
> 
> "zero_folio" would be confusing I'm afraid.
> 
> At least with current "is_zero_folio" etc.
> 
> "largest_zero_folio" or sth. like that might make it clearer that the size
> we are getting back might actually differ.
> 

That makes sense. I can change that in the next revision.

--
Pankaj