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

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Message-ID: <36bf71ea-f6aa-42fa-9fa2-53d00f351777@redhat.com>
Date: Fri, 12 Sep 2025 19:13:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kirill@...temov.name>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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 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 assume it doesn't really happen frequently, but could also happen 
after MADV_DONTNEED'ing the full range with pt reclaim I think after 
having faulted in some PTEs, for example.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb