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

[Barry Song <21cnbao@gmail.com>]

On Wed, Jun 25, 2025 at 11:27 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.06.25 13:15, Barry Song wrote:
> > On Wed, Jun 25, 2025 at 11:01 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 25.06.25 12:57, Barry Song wrote:
> >>>>>
> >>>>> Note that I don't quite understand why we have to batch the whole thing
> >>>>> or fallback to
> >>>>> individual pages. Why can't we perform other batches that span only some
> >>>>> PTEs? What's special
> >>>>> about 1 PTE vs. 2 PTEs vs. all PTEs?
> >>>>
> >>>> That's a good point about the "all-or-nothing" batching logic ;)
> >>>>
> >>>> It seems the "all-or-nothing" approach is specific to the lazyfree use
> >>>> case, which needs to unmap the entire folio for reclamation. If that's
> >>>> not possible, it falls back to the single-page slow path.
> >>>
> >>> Other cases advance the PTE themselves, while try_to_unmap_one() relies
> >>> on page_vma_mapped_walk() to advance the PTE. Unless we want to manually
> >>> modify pvmw.pte and pvmw.address outside of page_vma_mapped_walk(), which
> >>> to me seems like a violation of layers. :-)
> >>
> >> Please explain to me why the following is not clearer and better:
> >
> > This part is much clearer, but that doesn’t necessarily improve the overall
> > picture. The main challenge is how to exit the iteration of
> > while (page_vma_mapped_walk(&pvmw)).
>
> Okay, I get what you mean now.
>
> >
> > Right now, we have it laid out quite straightforwardly:
> >                  /* We have already batched the entire folio */
> >                  if (nr_pages > 1)
> >                          goto walk_done;
>
>
> Given that the comment is completely confusing whens seeing the check ... :)
>
> /*
>   * If we are sure that we batched the entire folio and cleared all PTEs,
>   * we can just optimize and stop right here.
>   */
> if (nr_pages == folio_nr_pages(folio))
>         goto walk_done;
>
> would make the comment match.

Yes, that clarifies it.

>
> >
> > with any nr between 1 and folio_nr_pages(), we have to consider two issues:
> > 1. How to skip PTE checks inside page_vma_mapped_walk for entries that
> > were already handled in the previous batch;
>
> They are cleared if we reach that point. So the pte_none() checks will
> simply skip them?
>
> > 2. How to break the iteration when this batch has arrived at the end.
>
> page_vma_mapped_walk() should be doing that?

It seems you might have missed the part in my reply that says:
"Of course, we could avoid both, but that would mean performing unnecessary
checks inside page_vma_mapped_walk()."

That’s true for both. But I’m wondering why we’re still doing the check,
even when we’re fairly sure they’ve already been cleared or we’ve reached
the end :-)

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.
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().

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry