linux-kernel - Re: [PATCH] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD

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Message-ID: <5n3vzdmdmeyicqf5dji2vo5hsfupigq7nl44cog7vwm6nchtqo@so5mvct4fwhn>
Date: Fri, 12 Sep 2025 18:44:43 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>, 
	Barry Song <baohua@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on
 SCAN_PMD_NULL

On Fri, Sep 12, 2025 at 07:13:48PM +0200, David Hildenbrand wrote:
> On 12.09.25 18:58, Kiryl Shutsemau wrote:
> > From: Kiryl Shutsemau <kas@...nel.org>
> > 
> > MADV_COLLAPSE on a file mapping behaves inconsistently depending on if
> > PMD page table is installed or not.
> > 
> > Consider following example:
> > 
> > 	p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
> > 		 MAP_SHARED, fd, 0);
> > 	err = madvise(p, 2UL << 20, MADV_COLLAPSE);
> > 
> > fd is a populated tmpfs file.
> > 
> > The result depends on the address that the kernel returns on mmap().
> > If it is located in an existing PMD table, the madvise() will succeed.
> > However, if the table does not exist, it will fail with -EINVAL.
> > 
> > This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when
> > a page table is missing, which causes collapse_pte_mapped_thp() to fail.
> > 
> > SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
> > collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
> > tables as needed.
> > 
> > Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> > ---
> 
> Makes sense to me.
> 
> Is this something we want a Fixes: or even Cc: stable for?

I am not sure if it is stable@ matter.

I believe it is there from the start:

Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")

-- 
  Kiryl Shutsemau / Kirill A. Shutemov