linux-kernel - Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

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Message-ID: <20240508155253.GK4650@nvidia.com>
Date: Wed, 8 May 2024 12:52:53 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: Lance Yang <ioworker0@...il.com>, Alistair Popple <apopple@...dia.com>,
	akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
	maskray@...gle.com, ryan.roberts@....com, david@...hat.com,
	21cnbao@...il.com, mhocko@...e.com, fengwei.yin@...el.com,
	zokeefe@...gle.com, shy828301@...il.com, xiehuan09@...il.com,
	libang.li@...group.com, wangkefeng.wang@...wei.com,
	songmuchun@...edance.com, peterx@...hat.com, minchan@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting
 into pagewalk loop

On Wed, May 08, 2024 at 10:56:34AM -0400, Zi Yan wrote:

> Lance is improving try_to_unmap_one() to support unmapping PMD THP as a whole,
> so he moves split_huge_pmd_address() inside while (page_vma_mapped_walk(&pvmw))
> and after mmu_notifier_invalidate_range_start() as split_huge_pmd_locked()
> and does not include the mmu notifier ops inside split_huge_pmd_address().
> I wonder if that could cause issues, since the mmu_notifier_invalidate_range_start()
> before the while loop only has range of the original address and
> split huge pmd can affect the entire PMD address range and these two ranges
> might not be the same.

That does not sound entirely good..

I suppose it depends on what split does, if the MM page table has the
same translation before and after split then perhaps no invalidation
is even necessary.

Jason