linux-kernel - Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

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Message-ID: <f66a7550-1e57-4a72-848b-e1c07c896818@lucifer.local>
Date: Tue, 6 May 2025 14:34:53 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org,
        Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
        pfalcato@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        david@...hat.com, peterx@...hat.com, ryan.roberts@....com,
        mingo@...nel.org, libang.li@...group.com, maobibo@...ngson.cn,
        zhengqi.arch@...edance.com, baohua@...nel.org, willy@...radead.org,
        ioworker0@...il.com, yang@...amperecomputing.com
Subject: Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

On Tue, May 06, 2025 at 02:40:44PM +0530, Anshuman Khandual wrote:
> On 5/6/25 10:30, Dev Jain wrote:
> > To use PTE batching, we want to determine whether the folio mapped by
> > the PTE is large, thus requiring the use of vm_normal_folio(). We want
> > to avoid the cost of vm_normal_folio() if the code path doesn't already
> > require the folio. For arm64, pte_batch_hint() does the job. To generalize
> > this hint, add a helper which will determine whether two consecutive PTEs
> > point to consecutive PFNs, in which case there is a high probability that
> > the underlying folio is large.
> >
> > Signed-off-by: Dev Jain <dev.jain@....com>
> > ---
> >  include/linux/pgtable.h | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index b50447ef1c92..28e21fcc7837 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -369,6 +369,22 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
> >  }
> >  #endif
> >
> > +/* Caller must ensure that ptep + 1 exists */
> > +static inline bool maybe_contiguous_pte_pfns(pte_t *ptep, pte_t pte)
> > +{
> > +	pte_t *next_ptep, next_pte;
> > +
> > +	if (pte_batch_hint(ptep, pte) != 1)
> > +		return true;
> > +
> > +	next_ptep = ptep + 1;
> > +	next_pte = ptep_get(next_ptep);
> > +	if (!pte_present(next_pte))
> > +		return false;
> > +
> > +	return unlikely(pte_pfn(next_pte) - pte_pfn(pte) == PAGE_SIZE);
> > +}
> > +
> >  #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> >  static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> >  					    unsigned long address,
>
> As mentioned earlier, this new helper maybe_contiguous_pte_pfns() does not
> have a proposed caller. Hence please do consider folding this forward with
> the next patch where move_ptes() starts using the helper. Besides, it is
> also difficult to review this patch without a proper caller context.

Agreed, please combine the two.