linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <2i5t62obfweid2zrt33vo3boviw4okha4d3gglw76eqv43ofky@pdv3evw5yjmh>
Date: Wed, 24 Sep 2025 11:17:00 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, baohua@...nel.org, 
	baolin.wang@...ux.alibaba.com, dev.jain@....com, hughd@...gle.com, ioworker0@...il.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, mpenttil@...hat.com, npache@...hat.com, 
	ryan.roberts@....com, ziy@...dia.com, richard.weiyang@...il.com
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries

On Wed, Sep 24, 2025 at 12:10:47PM +0200, David Hildenbrand wrote:
> On 24.09.25 12:02, Lance Yang wrote:
> > From: Lance Yang <lance.yang@...ux.dev>
> > 
> > The existing check in hpage_collapse_scan_pmd() is specific to uffd-wp
> > markers. Other special markers (e.g., GUARD, POISONED) would not be caught
> > early, leading to failures deeper in the swap-in logic.
> > 
> > hpage_collapse_scan_pmd()
> >   `- collapse_huge_page()
> >       `- __collapse_huge_page_swapin() -> fails!
> > 
> > As David suggested[1], this patch skips any such non-swap entries early.
> > If a special marker is found, the scan is aborted immediately with the
> > SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
> > work.
> 
> Note that I suggested to skip all non-present entries except swap entries,
> which includes migration entries, hwpoisoned entries etc.

Hm. So swap in is fine, but wait for migration to complete is not?
Seems odd.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov