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

[David Hildenbrand <david@redhat.com>]

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.

> 
> 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?

-- 
Cheers,

David / dhildenb