linux-kernel - Re: [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a huge page

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Date:   Tue, 17 Jan 2023 16:25:47 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a
 huge page

On Mon 09-01-23 12:53:13, Suren Baghdasaryan wrote:
> Protect VMA from concurrent page fault handler while collapsing a huge
> page. Page fault handler needs a stable PMD to use PTL and relies on
> per-VMA lock to prevent concurrent PMD changes. pmdp_collapse_flush(),
> set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
> not be detected by a page fault handler without proper locking.

I am struggling with this changelog. Maybe because my recollection of
the THP collapsing subtleties is weak. But aren't you just trying to say
that the current #PF handling and THP collapsing need to be mutually
exclusive currently so in order to keep that assumption you have mark
the vma write locked?

Also it is not really clear to me how that handles other vmas which can
share the same thp?
-- 
Michal Hocko
SUSE Labs