Re: [PATCH v7 13/16] mm: memory: support clearing page ranges

[David Hildenbrand]

On 17.09.25 23:44, Andrew Morton wrote:
> On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
>
> > Change folio_zero_user() to clear contiguous page ranges instead of
> > clearing using the current page-at-a-time approach. Exposing the largest
> > feasible length can be useful in enabling processors to optimize based
> > on extent.
>
> This patch is something which MM developers might care to take a closer
> look at.

I took a look at various revisions of this series, I'm only lagging 
behind on reviewing the latest series :)

> > However, clearing in large chunks can have two problems:
> >
> >   - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES)
> >     (larger folios don't have any expectation of cache locality).
> >
> >   - preemption latency when clearing large folios.
> >
> > Handle the first by splitting the clearing in three parts: the
> > faulting page and its immediate locality, its left and right
> > regions; with the local neighbourhood cleared last.
>
> Has this optimization been shown to be beneficial?
>
> If so, are you able to share some measurements?
>
> If not, maybe it should be removed?
>
> > ...
> >
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -7021,40 +7021,80 @@ static inline int process_huge_page(
> >   	return 0;
> >   }
> >   
> > -static void clear_gigantic_page(struct folio *folio, unsigned long addr_hint,
> > -				unsigned int nr_pages)
> > +/*
> > + * Clear contiguous pages chunking them up when running under
> > + * non-preemptible models.
> > + */
> > +static void clear_contig_highpages(struct page *page, unsigned long addr,
> > +				   unsigned int npages)
>
> Called "_highpages" because it wraps clear_user_highpages().  It really
> should be called clear_contig_user_highpages() ;)  (Not serious)

You have a point there, though :)

Fortunately this is only an internal helper.

--
Cheers

David / dhildenb