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

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Message-ID: <6179dd30-5351-4a79-b0d6-f0e85650a926@redhat.com>
Date: Wed, 25 Jun 2025 13:27:17 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Lance Yang <lance.yang@...ux.dev>, 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>
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
 folios during reclamation

On 25.06.25 13:15, Barry Song wrote:
> On Wed, Jun 25, 2025 at 11:01 PM David Hildenbrand <david@...hat.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.

> 
> 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?

-- 
Cheers,

David / dhildenb