Re: [PATCH RFC v2] mm: introduce copy_mc_highpages() and batch copy in folio_mc_copy()

[Garg, Shivank]

On Mon, 2026-10-19 at 14:23 +0200, David Hildenbrand (Arm) wrote:
> > > +}
> > > +#else
> > > +
> > > +#define COPY_MC_PAGES_NON_PREEMPT_BATCH		1
> > 
> > I would suggest that perhaps this should be the fallback for non-preemptible
> > models. But it gets a little weird with PREEMPT_DYNAMIC I think?
> 
> Why can't we just use PROCESS_PAGES_NON_PREEMPT_BATCH where we have similar
> handling already?
> 
> > 

I did not reuse PROCESS_PAGES_NON_PREEMPT_BATCH because it depends on
whether clear_pages is defined. While COPY_MC_PAGES_NON_PREEMPT_BATCH
depends on copy_mc_to_kernel and other constraints. So, using a single
param for both would make it more messy.

However as Pedro pointed, most architectures are full preempt/preempt lazy,
and with this commit 7dadeaa6e851 ("sched: Further restrict the
preemption modes"), for PREEMPT_DYNAMIC, preempt=none and preempt=voluntary
are rejected at boot. preempt_model_preemptible() is therefore always
true for this case.

So, this can be simplfied to drop the COPY_MC_PAGES_NON_PREEMPT_BATCH.


> > > +
> > > +static inline int copy_mc_highpages(struct page *to, struct page *from,
> > > +		unsigned long nr_pages)
> > > +{
> > > +	unsigned long i;
> > > +
> > > +	for (i = 0; i < nr_pages; i++)
> > > +		if (copy_mc_highpage(to + i, from + i))
> > > +			return -EHWPOISON;
> > > +	return 0;
> > > +}
> > > +#endif
> > > +
> > >  static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> > >  			       struct page *src_page, size_t src_off,
> > >  			       size_t len)
> > > diff --git a/mm/util.c b/mm/util.c
> > > index 7f374bb8bf36..5015cfdfeaa5 100644
> > > --- a/mm/util.c
> > > +++ b/mm/util.c
> > > @@ -763,13 +763,23 @@ EXPORT_SYMBOL(folio_copy);
> > >  
> > > 
> > >  int folio_mc_copy(struct folio *dst, struct folio *src)
> > >  {
> > > -	long nr = folio_nr_pages(src);
> > > -	long i = 0;
> > > +	const unsigned long nr_pages = folio_nr_pages(src);
> > > +	/*
> > > +	 * Copy the largest extent possible to allow for architecture specific
> > > +	 * extent based optimizations. copy_mc_highpages() does not call
> > > +	 * cond_resched(), so limit the unit size when running under
> > > +	 * non-preemptible scheduling models.
> > > +	 */
> > > +	const unsigned long unit = preempt_model_preemptible() ?
> > > +				   nr_pages : COPY_MC_PAGES_NON_PREEMPT_BATCH;
> > 
> > See? now mm is aware of preemption models! eww!
> 
> Yeah, we have that in clear_contig_highpages() already, which is getting
> replicated here for copying instead of zeroing.