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

On Wed, Jun 25, 2025 at 11:27 PM David Hildenbrand [off-list ref] wrote:

On 25.06.25 13:15, Barry Song wrote:

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On Wed, Jun 25, 2025 at 11:01 PM David Hildenbrand [off-list ref] wrote:

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On 25.06.25 12:57, Barry Song wrote:

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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.

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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.

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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?

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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