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

[David Hildenbrand <david@redhat.com>]

On 25.06.25 13:42, Barry Song wrote:
> 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.

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.

-- 
Cheers,

David / dhildenb