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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-Archived-By: 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 X-Spam-Status: No, score=-9.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam09 X-Stat-Signature: gkf34rf6pt5fpzc6a3z7uc9f4y9n6fkw X-Rspamd-Queue-Id: D424B14000E X-Rspam-User: X-HE-Tag: 1794415957-352994 X-HE-Meta: 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 iDDe+ZaB 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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