linux-kernel - Re: [RFC PATCH 3/3] mm/mremap: Use can_pte_batch_count() instead of folio_pte

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Message-ID: <8dc1d253056848dba950e74bed8218bd@huawei.com>
Date: Tue, 28 Oct 2025 13:27:09 +0000
From: zhangqilong <zhangqilong3@...wei.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
CC: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"david@...hat.com" <david@...hat.com>, "Liam.Howlett@...cle.com"
	<Liam.Howlett@...cle.com>, "vbabka@...e.cz" <vbabka@...e.cz>,
	"rppt@...nel.org" <rppt@...nel.org>, "surenb@...gle.com" <surenb@...gle.com>,
	"mhocko@...e.com" <mhocko@...e.com>, "jannh@...gle.com" <jannh@...gle.com>,
	"pfalcato@...e.de" <pfalcato@...e.de>, "linux-mm@...ck.org"
	<linux-mm@...ck.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Wangkefeng (OS Kernel Lab)"
	<wangkefeng.wang@...wei.com>, Sunnanyong <sunnanyong@...wei.com>
Subject: Re: [RFC PATCH 3/3] mm/mremap: Use can_pte_batch_count() instead of
 folio_pte_batch() for pte batch

> On Mon, Oct 27, 2025 at 10:03:15PM +0800, Zhang Qilong wrote:
> > In current mremap_folio_pte_batch(), 1) pte_batch_hint() always return
> > one pte in non-ARM64 machine, it is not efficient. 2) Next,
> 
> Err... but there's basically no benefit for non-arm64 machines?

It except have benefit in non-arm64 machines. In non-arm64 machine,
pte_batch_hint always return one although a folio are mapped by multiple
PTEs. 

> 
> The key benefit is the mTHP side of things and making the underlying arch-
> specific code more efficient right?

Yes, we except it could benefit from mTHP, and not just for arm64.

> 
> And again you need to get numbers to demonstrate you don't regress non-
> arm64.

Yes, I will have a test on x86-64, non-contiguous folios or non-contiguous-folio
should not cause regression. Thanks for your kind reminder.

> 
> > it need to acquire a folio to call the folio_pte_batch().
> >
> > Due to new added can_pte_batch_count(), we just call it instead of
> > folio_pte_batch(). And then rename mremap_folio_pte_batch() to
> > mremap_pte_batch().
> >
> > Signed-off-by: Zhang Qilong <zhangqilong3@...wei.com>
> > ---
> >  mm/mremap.c | 16 +++-------------
> >  1 file changed, 3 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c index
> > bd7314898ec5..d11f93f1622f 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -169,27 +169,17 @@ static pte_t move_soft_dirty_pte(pte_t pte)
> >  		pte = pte_swp_mksoft_dirty(pte);
> >  #endif
> >  	return pte;
> >  }
> >
> > -static int mremap_folio_pte_batch(struct vm_area_struct *vma,
> > unsigned long addr,
> > +static int mremap_pte_batch(struct vm_area_struct *vma, unsigned long
> > +addr,
> >  		pte_t *ptep, pte_t pte, int max_nr)  {
> > -	struct folio *folio;
> > -
> >  	if (max_nr == 1)
> >  		return 1;
> >
> > -	/* Avoid expensive folio lookup if we stand no chance of benefit. */
> > -	if (pte_batch_hint(ptep, pte) == 1)
> > -		return 1;
> 
> Why are we eliminating an easy exit here and instead always invoking the more
> involved function?
> 
> Again this has to be tested against non-arm architectures.
> 
> > -
> > -	folio = vm_normal_folio(vma, addr, pte);
> > -	if (!folio || !folio_test_large(folio))
> > -		return 1;
> > -
> > -	return folio_pte_batch(folio, ptep, pte, max_nr);
> > +	return can_pte_batch_count(vma, ptep, &pte, max_nr, 0);
> 
> This is very silly to have this function now ust return another function + a trivial
> check that your function should be doing...
> 
> >  }
> >
> >  static int move_ptes(struct pagetable_move_control *pmc,
> >  		unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
> { @@ -278,11
> > +268,11 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >  		 * make sure the physical page stays valid until
> >  		 * the TLB entry for the old mapping has been
> >  		 * flushed.
> >  		 */
> >  		if (pte_present(old_pte)) {
> > -			nr_ptes = mremap_folio_pte_batch(vma, old_addr,
> old_ptep,
> > +			nr_ptes = mremap_pte_batch(vma, old_addr, old_ptep,
> >  							 old_pte,
> max_nr_ptes);
> >  			force_flush = true;
> >  		}
> >  		pte = get_and_clear_ptes(mm, old_addr, old_ptep, nr_ptes);
> >  		pte = move_pte(pte, old_addr, new_addr);
> > --
> > 2.43.0
> >