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

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Message-ID: <CAGsJ_4y1GObH-C7R=FQL=UWe3kF6qhKoRqPxNPYx0k7uwocc+g@mail.gmail.com>
Date: Wed, 25 Jun 2025 22:57:27 +1200
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: David Hildenbrand <david@...hat.com>, 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

> >
> > 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. :-)

Thanks
Barry