[PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation
David Hildenbrand
david at redhat.com
Wed Jun 25 05:25:24 PDT 2025
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
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