linux-kernel - Re: [PATCH v3 1/5] mm: rmap: support batched checks of the references for large folios

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Message-ID: <aUV4u9pOHxvQiDcP@casper.infradead.org>
Date: Fri, 19 Dec 2025 16:09:31 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	akpm@...ux-foundation.org, david@...nel.org,
	catalin.marinas@....com, will@...nel.org,
	lorenzo.stoakes@...cle.com, ryan.roberts@....com, vbabka@...e.cz,
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
	riel@...riel.com, harry.yoo@...cle.com, jannh@...gle.com,
	baohua@...nel.org, dev.jain@....com, linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] mm: rmap: support batched checks of the
 references for large folios

On Fri, Dec 19, 2025 at 10:47:52AM -0500, Liam R. Howlett wrote:
> > -#define ptep_clear_flush_young_notify(__vma, __address, __ptep)		\
> > +#define ptep_clear_flush_young_notify(__vma, __address, __ptep, __nr)	\
> >  ({									\
> >  	int __young;							\
> >  	struct vm_area_struct *___vma = __vma;				\
> >  	unsigned long ___address = __address;				\
> > -	__young = ptep_clear_flush_young(___vma, ___address, __ptep);	\
> > +	unsigned int ___nr = __nr;					\
> > +	__young = clear_flush_young_ptes(___vma, ___address, __ptep, ___nr);	\
> >  	__young |= mmu_notifier_clear_flush_young(___vma->vm_mm,	\
> >  						  ___address,		\
> >  						  ___address +		\
> > -							PAGE_SIZE);	\
> > +						nr * PAGE_SIZE);	\
> 
> Did you mean nr * PAGE_SIZE here?  I think it should be __nr or ___nr?
> I think this nr variable works because it exists where this macro is
> expanded?

Yes, this should clearly be ___nr.

> I am also not sure why you have ___nr  at all?

It's a macro cleanliness thing.  Imagine that we have a caller:

	a = ptep_clear_flush_young_notify(vma, addr, ptep, nr++);

If you have two references to the __nr macro argument, then you end up
incrementing nr twice.  Assigning __nr to ___nr and then referring to
___nr within the macro prevents this.

That said, I'm not sure why ptep_clear_flush_young_notify() needs
to be a macro instead of a static inline?