linux-kernel - Re: [RFC 2/3] mm: add STATIC_PMD_ZERO

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Message-ID: <jewtporls43r5y3eybqzm4bcku5sf3wzw6ewfjbyykeb3mxp27@ydjcrck6ldkd>
Date: Tue, 27 May 2025 21:28:35 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Pankaj Raghav <p.raghav@...sung.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Vlastimil Babka <vbabka@...e.cz>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, 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, linux-block@...r.kernel.org, willy@...radead.org, x86@...nel.org, 
	linux-fsdevel@...r.kernel.org, "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org, 
	gost.dev@...sung.com, hch@....de
Subject: Re: [RFC 2/3] mm: add STATIC_PMD_ZERO_PAGE config option

> > You are right that if this config is disabled, the callers with NULL mm
> > struct are guaranteed to fail, but we are not generating extra code
> > because there are still users who want dynamic allocation.
> 
> I'm pretty sure you're making the compiler generate unnecessary code.
> Think of this:
> 
> 	if (mm_get_huge_zero_folio(mm)
> 		foo();
> 	else
> 		bar();
> 
> With the static zero page, foo() is always called. But bar() is dead
> code. The compiler doesn't know that, so it will generate both sides of
> the if().
> 

Ahh, yeah you are right. I was thinking about the callee and not the
caller.

> If you can get the CONFIG_... option checks into the header, the
> compiler can figure it out and not even generate the call to bar().

Got it. I will keep this in mind before sending the next version.

> > Do you think it is better to have the code with inside an #ifdef instead
> > of using the IS_ENABLED primitive?
> It has nothing to do with an #ifdef versus IS_ENABLED(). It has to do
> with the compiler having visibility into how mm_get_huge_zero_folio()
> works enough to optimize its callers.

I think something like this should give some visibility to the compiler:

struct folio *huge_zero_folio __read_mostly;

...
#if CONFIG_STATIC_PMD_ZERO_PAGE

struct folio* mm_get_huge_zero_folio(...)
{
  return READ_ONCE(huge_zero_folio);
}

#else

struct folio* mm_get_huge_zero_folio(...)
{
  <old-code>
}

#endif

But I am not sure here if the compiler can assume here the static
huge_zero_folio variable will be non-NULL. It will be interesting to
check that in the output.

--
Pankaj