Re: [PATCH v7 3/3] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[David Hildenbrand]

> > +			nr = madvise_folio_pte_batch(addr, end, folio, pte,
> > +						     ptent, &any_young, &any_dirty);
> > +
> > +			if (nr < folio_nr_pages(folio)) {
> > +				if (folio_likely_mapped_shared(folio))
> > +					continue;
> > +
> > +				arch_leave_lazy_mmu_mode();
> > +				if (madvise_pte_split_folio(mm, pmd, addr,
> > +							    folio, &start_pte, &ptl))
> > +					nr = 0;
> > +				if (!start_pte)
> > +					break;
> > +				pte = start_pte;
> > +				arch_enter_lazy_mmu_mode();
> > +				continue;
> > +			}
> > +
> > +			if (any_young)
> > +				ptent = pte_mkyoung(ptent);
> > +			if (any_dirty)
> > +				ptent = pte_mkdirty(ptent);
> >   		}
> >   
> > +		if (folio_mapcount(folio) != folio_nr_pages(folio))
> > +			continue;
>
> Why is this here? I thought we had previously concluded to only do this test
> inside the below if statement (where you have it duplicated).

I stumbled over these same while reviewing. It's not exactly duplicate, 
because it's unreliable without the folio lock. It looks more like an 
best-effort early check.

But then, we also add it to cases where we previously wouldn't check the 
mapcount at all: when the folio was added to the swapcache or is already 
dirty.

In that case, we would even see a change for order-0 folios with that 
new check.

--
Cheers,

David / dhildenb