Message-ID: <16d6642b-6f22-4d7b-94d6-92692b63cbd9@lucifer.local>
Date: Wed, 17 Dec 2025 14:50:50 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...nel.org, catalin.marinas@....com,
        will@...nel.org, ryan.roberts@....com, Liam.Howlett@...cle.com,
        vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
        riel@...riel.com, harry.yoo@...cle.com, jannh@...gle.com,
        willy@...radead.org, baohua@...nel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] arm64: mm: support batch clearing of the young
 flag for large folios

On Wed, Dec 17, 2025 at 11:53:31AM +0800, Baolin Wang wrote:
>
>
> On 2025/12/16 19:11, Lorenzo Stoakes wrote:
> > On Tue, Dec 16, 2025 at 11:32:58AM +0800, Baolin Wang wrote:
> > >
> > >
> > > On 2025/12/15 19:36, Lorenzo Stoakes wrote:
> > > > On Thu, Dec 11, 2025 at 04:16:54PM +0800, Baolin Wang wrote:
> > > > > Currently, contpte_ptep_test_and_clear_young() and contpte_ptep_clear_flush_young()
> > > > > only clear the young flag and flush TLBs for PTEs within the contiguous range.
> > > > > To support batch PTE operations for other sized large folios in the following
> > > > > patches, adding a new parameter to specify the number of PTEs.
> > > > >
> > > > > While we are at it, rename the functions to maintain consistency with other
> > > > > contpte_*() functions.
> > > > >
> > > > > Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > > > > ---
> > > > >    arch/arm64/include/asm/pgtable.h | 12 ++++-----
> > > > >    arch/arm64/mm/contpte.c          | 44 ++++++++++++++++++++++----------
> > > > >    2 files changed, 37 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > > > > index 0944e296dd4a..e03034683156 100644
> > > > > --- a/arch/arm64/include/asm/pgtable.h
> > > > > +++ b/arch/arm64/include/asm/pgtable.h
> > > > > @@ -1679,10 +1679,10 @@ extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> > > > >    extern pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
> > > > >    				unsigned long addr, pte_t *ptep,
> > > > >    				unsigned int nr, int full);
> > > > > -extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> > > > > -				unsigned long addr, pte_t *ptep);
> > > > > -extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> > > > > -				unsigned long addr, pte_t *ptep);
> > > > > +extern int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
> > > > > +				unsigned long addr, pte_t *ptep, unsigned int nr);
> > > > > +extern int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
> > > > > +				unsigned long addr, pte_t *ptep, unsigned int nr);
> > > >
> > > > In core mm at least, as a convention, we strip 'extern' from header declarations
> > > > as we go as they're not necessary.
> > >
> > > Sure. I can remove the 'extern'.
> >
> > Thanks.
> >
> > >
> > > > I wonder about this naming convention also. The contpte_xxx_() prefix
> > > > obviously implies we are operating upon PTEs in the contiguous range, now
> > > > we are doing something... different and 'nr' is a bit vague.
> > > >
> > > > Is it the nr of contiguous ranges? Well in reality it's nr_pages right? But
> > >
> > > Yes, 'nr' here means nr_pages, which follows the convention of the
> > > parameters in contpte_xxx_() functions.
> >
> > Except you're violating the convention already by doing non-contpte stuff in
> > contpte functions?
>
> Nope. Sorry for my poor English, which might have caused some confusion. I
> followed the contpte's convention, and let me try to explain it again below.
>
> > The confusion is between nr of contpte ranges and nr of pages, so I think we
> > should be explicit.
> >
> > >
> > > > now we're not saying they're necessarily contiguous in the sense of contpte...
> > > >
> > > > I wonder whether we'd be better off introducing a new function that
> > > > explicitly has 'batch' or 'batched' in the name, and have the usual
> > > > contpte_***() stuff and callers that need batching using a new underlying helper?
> > >
> > > OK. I get your point. However, this is outside the scope of my patchset.
> >
> > Well, no, this is review feedback that needs to be addressed, I really don't
> > like this patch as-is.
> >
> > So for this to be upstreamable you really need to separate this out.
> >
> > >
> > > Perhaps we can clean up all the contpte_xxx_() functions if everyone agrees
> > > that this is confusing.
> >
> > I mean, unless Ryan explicitly makes a case for this being fine, I'm inclined to
> > reject the patch unless we do this.
> >
> > The contpte logic is already very confusing, and this is only adding to it.
> >
> > >
> > > > Obviously I defer to Ryan on all this, just my thoughts...
> > > >
> > > > >    extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> > > > >    				pte_t *ptep, unsigned int nr);
> > > > >    extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> > > > > @@ -1854,7 +1854,7 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> > > > >    	if (likely(!pte_valid_cont(orig_pte)))
> > > > >    		return __ptep_test_and_clear_young(vma, addr, ptep);
> > > > >
> > > > > -	return contpte_ptep_test_and_clear_young(vma, addr, ptep);
> > > > > +	return contpte_test_and_clear_young_ptes(vma, addr, ptep, CONT_PTES);
> > > > >    }
> > > > >
> > > > >    #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
> > > > > @@ -1866,7 +1866,7 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
> > > > >    	if (likely(!pte_valid_cont(orig_pte)))
> > > > >    		return __ptep_clear_flush_young(vma, addr, ptep);
> > > > >
> > > > > -	return contpte_ptep_clear_flush_young(vma, addr, ptep);
> > > > > +	return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
> > > > >    }
> > > > >
> > > > >    #define wrprotect_ptes wrprotect_ptes
> > > > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > > > > index c0557945939c..19b122441be3 100644
> > > > > --- a/arch/arm64/mm/contpte.c
> > > > > +++ b/arch/arm64/mm/contpte.c
> > > > > @@ -488,8 +488,9 @@ pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(contpte_get_and_clear_full_ptes);
> > > > >
> > > > > -int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> > > > > -					unsigned long addr, pte_t *ptep)
> > > > > +int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
> > > > > +					unsigned long addr, pte_t *ptep,
> > > > > +					unsigned int nr)
> > > > >    {
> > > > >    	/*
> > > > >    	 * ptep_clear_flush_young() technically requires us to clear the access
> > > > > @@ -500,39 +501,56 @@ int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> > > > >    	 * having to unfold.
> > > > >    	 */
> > > >
> > > > Hmm shouldn't you need to update this comment now 'nr' is a thing? E.g.:
> > > >
> > > > "And since we only create a contig range when the range is covered by a single
> > > > folio, we can get away with clearing young for the whole contig range here, so
> > > > we avoid having to unfold."
> > >
> > > I think the original comments are still clear:
> > > "
> > > /*
> > >   * ptep_clear_flush_young() technically requires us to clear the access
> > >   * flag for a _single_ pte. However, the core-mm code actually tracks
> > >   * access/dirty per folio, not per page. And since we only create a
> > >   * contig range when the range is covered by a single folio, we can get
> > >   * away with clearing young for the whole contig range here, so we avoid
> > >   * having to unfold.
> > >   */
> > > "
> >
> > Except nr means you can span more than a single contig pte range? So this
> > comment is no longer applicable as-is?
>
> The nr means consecutive (present) PTEs that map consecutive pages of the
> same large folio in a single VMA and a single page table.
>
> I think this is a prerequisite. If you think it's necessary to explain it, I
> can add it to the comments.
>
> > You've changed the function, the comment needs to change too.
> >
> > >
> > > > However now you're allowing for large folios that are not contpte?
> > > >
> > > > Overall feeling pretty iffy about implementing this this way.
> > >
> > > Please see the above comments, which explain why we should do that.
> >
> > You haven't explained anything? Maybe I missed it?
> >
> > Can you explain clearly what nr might actually be in relation to contpte's? I'm
> > confused even as to the utility here.
> >
> > Is it batched ranges of contptes? But then why are you trying to see if end is a
> > contpte + expand the range if so?
>
> See below.
>
> > > > > +	unsigned long start = addr;
> > > > > +	unsigned long end = start + nr * PAGE_SIZE;
> > > >
> > > > So now [addr, addr + nr * PAGE_SIZE) must for sure be within a single PTE
> > > > table?
> > >
> > > Caller has made sure that (see folio_referenced_one()).
> >
> > You should comment this somehow, I hate this nested assumptions stuff 'function
> > X which calls function Y which calls function Z makes sure that parameter A
> > fulfils requirements B but we don't mention it anywhere'.
>
> Sure.
>
> > > > >    	int young = 0;
> > > > >    	int i;
> > > > >
> > > > > -	ptep = contpte_align_down(ptep);
> > > > > -	addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> > > > > +	if (pte_cont(__ptep_get(ptep + nr - 1)))
> > > > > +		end = ALIGN(end, CONT_PTE_SIZE);
> > > >
> > > > OK so I guess for PTE_CONT to be set, it must be aligned to CONT_PTE_SIZE,
> > > > with other PTE entries present there not having PTE_CONT set (Ryan - do
> > > > correct me if I'm wrong!)
> > > >
> > > > I guess then no worry about running off the end of the PTE table here.
> > > >
> > > > I wonder about using pte_cont_addr_end() here, but I guess you are not
> > > > intending to limit to a range here but rather capture the entire contpte
> > > > range of the end.
> > >
> > > Right.
> > >
> > > > I don't love that now a function prefixed with contpte_... can have a
> > > > condition where part of the input range is _not_ a contpte entry, or is
> > > > specified partially...
> > >
> > > Like I said above, this is outside the scope of my patchset. If Ryan also
> > > thinks we need to do some cleanups for all the contpte_xxx() functions in
> > > the contpte.c file, we can send a follow-up patchset to rename all the
> > > contpte_xxx() functions to make it clear.
> >
> > It's really not outside of the scope of the patch set, this is review feedback
> > you have to address :) trying not to be a grumpy kernel dev here :>)
> >
> > In general in the kernel we don't accept confusing patches then defer making
> > them not-confusing until later.
> >
> > We review until the series is at the correct standard to be accepted before we
> > merge.
> >
> > As always I'm happy to be convinced that I'm mistaken or something's not
> > necesary, however!
>
> Yes, let me try to explain it again. See below.
>
> > > > > -	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> > > > > -		young |= __ptep_test_and_clear_young(vma, addr, ptep);
> > > > > +	if (pte_cont(__ptep_get(ptep))) {
> > > > > +		start = ALIGN_DOWN(start, CONT_PTE_SIZE);
> > > > > +		ptep = contpte_align_down(ptep);
> > > > > +	}
> > > > > +
> > > > > +	nr = (end - start) / PAGE_SIZE;
> > > >
> > > > Hm don't love this reuse of input param.
> > >
> > > OK. Will use another local variable instead.
> >
> > Thanks.
> >
> > >
> > > >
> > > > > +	for (i = 0; i < nr; i++, ptep++, start += PAGE_SIZE)
> > > > > +		young |= __ptep_test_and_clear_young(vma, start, ptep);
> > > >
> > > > Again, overall find it a bit iffy that we are essentially overloading the
> > > > code with non-contpte behaviour.
> > >
> > > Let me clarify further: this is the handling logic of the contpte_xxx()
> > > functions in the contpte.c file. For example, the function
> > > contpte_set_ptes() has a parameter 'nr', which may be greater than
> > > CONT_PTES, meaning that it can handle multiple CONT_PTES range sizes of a
> > > large folio.
> > >
> > > It might be a bit confusing, and I understand this is why you want to change
> > > the 'contpte_' prefix to the 'batch_' prefix. Of course, we can hope Ryan
> > > can provide some input on this.
> > >
> > > Thanks for reviewing.
> >
> > OK so we have some reeeeal confusion here. And this speaks to the patch needing
> > work.
> >
> > This seems to answer what I asked above - basically - are we doing _batches_ of
> > _contpte_ entries, or are we just accepting arbitrary large folios here and
> > batching up operations on them, including non-contpte entries.
> >
> > Presumably from the above you're saying - this is dealing with batches of
> > contpte entries.
> >
> > In which case this has to be made _super_ clear. Not just adding an arbitrary
> > 'nr' parameter and letting people guess.
> >
> > The contpte code is _already_ confusing and somewhat surprising if you're not
> > aware of it, we need to be going in the other direction.
>
> Sure. Sorry for causing confusion. I try to explain it again to make it
> clear.
>
> First, it is necessary to explain how the contpte implementation in Arm64 is
> exposed, using set_ptes() as an example.
>
> Arm64 defines an arch-specific implementation of set_ptes():
> "
> static __always_inline void set_ptes(struct mm_struct *mm, unsigned long
> addr,
> 				pte_t *ptep, pte_t pte, unsigned int nr)
> {
> 	pte = pte_mknoncont(pte);
>
> 	if (likely(nr == 1)) {
> 		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> 		__set_ptes(mm, addr, ptep, pte, 1);
> 		contpte_try_fold(mm, addr, ptep, pte);
> 	} else {
> 		contpte_set_ptes(mm, addr, ptep, pte, nr);
> 	}
> }
> "
>
> Here, 'nr' refers to consecutive (present) PTEs that map consecutive pages
> of the same large folio within a single VMA and a single page table. Based
> on 'nr', it determines whether the PTE_CONT attribute should be set or not.
>
> Second, the underlying implementation of contpte_set_ptes() also calculates
> 'end' based on 'nr' and checks whether the PTE_CONT attribute should be set.
> For example:
>
> For a large folio with order = 3 (nr = 8), the PTE_CONT attribute will not
> be set since the 'next' is not aligned with CONT_PTE_MASK.
>
> For a large folio with order = 4 (nr = 16), it aligns perfectly, allowing
> the PTE_CONT attribute to be set for the entire PTE entries of the large
> folio.
>
> For a large folio with order = 5 (nr = 32), this involves two ranges of
> CONT_PTE_SIZE (16 PTEs in each range), requiring the PTE_CONT attribute to
> be set separately for each range.
>
> "
> void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> 					pte_t *ptep, pte_t pte, unsigned int nr)
> {
> 	.......
>
> 	end = addr + (nr << PAGE_SHIFT);
> 	pfn = pte_pfn(pte);
> 	prot = pte_pgprot(pte);
>
> 	do {
> 		next = pte_cont_addr_end(addr, end);
> 		nr = (next - addr) >> PAGE_SHIFT;
> 		pte = pfn_pte(pfn, prot);
>
> 		if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0)
> 			pte = pte_mkcont(pte);
> 		else
> 			pte = pte_mknoncont(pte);
>
> 		__set_ptes(mm, addr, ptep, pte, nr);
>
> 		addr = next;
> 		ptep += nr;
> 		pfn += nr;
>
> 	} while (addr != end);
> }
> "
>
> Third, regarding the implementation of my patch, I have followed the contpte
> logic here by adding the 'nr' parameter to
> contpte_ptep_test_and_clear_young() for batch processing consecutive
> (present) PTEs. Moreover, the semantics of 'nr' remain consistent with the
> original contpte functions, and I have not broken the existing contpte
> implementation logic.

OK that helps a lot thanks.

So the nr will be number of PTEs that are consecutive, belong to the same folio,
and have the same PTE bit set excluding accessed, writable, dirty, soft-dirty.

But since we are retrieving accessed bit state (young), isn't it a problem we're
ignoring this bit when considering what goes into the batch?

I mean now it's going to return true even if some do not have the accessed flag
set?

I am more convinced things are correct in style at least then this way as, as
you say, this is the approach taken in set_ptes().

Thanks, Lorenzo

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