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

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Message-ID: <9bb1e917-891d-4e1b-915f-98cdd5fc578b@redhat.com>
Date: Wed, 25 Jun 2025 14:25:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>
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 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