Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[David Hildenbrand]

On 25.06.25 14:20, Lance Yang wrote:
> On 2025/6/25 20:09, David Hildenbrand wrote:
> > > Somehow, I feel we could combine your cleanup code—which handles a batch
> > > size of "nr" between 1 and nr_pages—with the
> > > "if (nr_pages == folio_nr_pages(folio)) goto walk_done" check.
> >
> > Yeah, that's what I was suggesting. It would have to be part of the
> > cleanup I think.
> >
> > I'm still wondering if there is a case where
> >
> > if (nr_pages == folio_nr_pages(folio))
> >       goto walk_done;
> >
> > would be wrong when dealing with small folios.
> >
> > > In practice, this would let us skip almost all unnecessary checks,
> > > except for a few rare corner cases.
> > >
> > > For those corner cases where "nr" truly falls between 1 and nr_pages,
> > > we can just leave them as-is—performing the redundant check inside
> > > page_vma_mapped_walk().
> >
> > I mean, batching mapcount+refcount updates etc. is always a win. If we
> > end up doing some unnecessary pte_none() checks, that might be
> > suboptimal but mostly noise in contrast to the other stuff we will
> > optimize out 🙂
> >
> > Agreed that if we can easily avoid these pte_none() checks, we should do
> > that. Optimizing that for "nr_pages == folio_nr_pages(folio)" makes sense.
>
> Hmm... I have a question about the reference counting here ...
>
> 		if (vma->vm_flags & VM_LOCKED)
> 			mlock_drain_local();
> 		folio_put(folio);
> 		/* We have already batched the entire folio */
>
> Does anyone else still hold a reference to this folio after folio_put()?

The caller of the unmap operation should better hold a reference :)

Also, I am not sure why we don't perform a

folio_put_refs(folio, nr_pages);

... :)

--
Cheers,

David / dhildenb