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

[Lance Yang <lance.yang@linux.dev>]

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()?

		if (nr_pages == folio_nr_pages(folio))
			goto walk_done;
		continue;
walk_abort:
		ret = false;
walk_done:
		page_vma_mapped_walk_done(&pvmw);
		break;
	}

Thanks,
Lance


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