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Fri, 12 Sep 2025 13:44:45 -0400 (EDT) Date: Fri, 12 Sep 2025 18:44:43 +0100 From: Kiryl Shutsemau To: David Hildenbrand Cc: Andrew Morton , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL Message-ID: <5n3vzdmdmeyicqf5dji2vo5hsfupigq7nl44cog7vwm6nchtqo@so5mvct4fwhn> References: <36bf71ea-f6aa-42fa-9fa2-53d00f351777@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <36bf71ea-f6aa-42fa-9fa2-53d00f351777@redhat.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2C4D41C000D X-Stat-Signature: ffa4ii86z6irgu5wrbatz8bd11xcse6t X-Rspam-User: X-HE-Tag: 1757699088-266162 X-HE-Meta: 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 snWSyk/6 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > > > 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 > > --- > > 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