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[83.60.147.6]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-498363dffa1sm11516874f8f.39.2026.11.11.08.52.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 11 Nov 2026 08:52:35 -0800 (PST) Message-ID: <9d4ce24b-7799-4d71-84ae-983028366ae1@gmail.com> Date: Wed, 11 Nov 2026 17:52:33 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] mm: drop the huge PMD assertion from walk_to_pmd() To: Eliav Farber , Andrew Morton Cc: David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , x86@kernel.org, William Kucharski , Vishal Moola , Alex Williamson , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261111140655.34281-1-farbere@amazon.com> <20261111140655.34281-2-farbere@amazon.com> Content-Language: en-US From: "Jose A. Perez de Azpillaga" In-Reply-To: <20261111140655.34281-2-farbere@amazon.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/11/26 3:06 PM, Eliav Farber wrote: > walk_to_pmd() asserts that the PMD it returns is not a huge entry. That > held when commit c2febafc6773 ("mm: convert generic code to 5-level > paging") added the assertion: nothing installed a huge PMD over the > mappings its callers walk. > > ARCH_SUPPORTS_HUGE_PFNMAP changed that. A VM_PFNMAP mapping whose driver > provides both ->fault and ->huge_fault can have a PMD leaf installed by a > concurrent huge fault over the same address, so a caller can observe one > and BUG on CONFIG_DEBUG_VM=y. > > The walk does not need the invariant. pmd_alloc() only allocates when the > PUD is empty and otherwise returns pmd_offset(), so a leaf is returned > rather than overwritten. Every caller already copes with one, because > they all reach pte_offset_map_lock(), which refuses to walk a leaf and > returns NULL: insert_pages() turns that into -EFAULT, get_locked_pte() > passes the NULL to its callers, and insert_pfn() returns a fault code. > > Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support") > Signed-off-by: Eliav Farber LGTM: Reviewed-by: Jose A. Perez de Azpillaga -- cheers, jose a. p-a