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

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Message-ID: <42f1d84f-2d17-43b7-8fa2-83322fcca44f@linux.dev>
Date: Wed, 25 Jun 2025 20:20:59 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, baolin.wang@...ux.alibaba.com,
 chrisl@...nel.org, kasong@...cent.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, lorenzo.stoakes@...cle.com,
 ryan.roberts@....com, v-songbaohua@...o.com, x86@...nel.org,
 ying.huang@...el.com, zhengtangquan@...o.com,
 Lance Yang <ioworker0@...il.com>, Barry Song <21cnbao@...il.com>
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
 folios during reclamation



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