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Tue, 25 Nov 2025 11:21:18 -0800 (PST) X-Google-Smtp-Source: AGHT+IGCwC+zV9ilL/kZYBgogz1BW/k0XjgDzjkTt78Ehqb7OwOZ4OSkjcxPe6SUz84vf0wVoVyZhQ== X-Received: by 2002:a05:620a:700b:b0:891:9bb6:6b9c with SMTP id af79cd13be357-8b33d4cfb76mr2360721485a.44.1764098478205; Tue, 25 Nov 2025 11:21:18 -0800 (PST) Received: from x1.local ([142.188.210.156]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8b32953735dsm1226353585a.24.2025.11.25.11.21.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Nov 2025 11:21:17 -0800 (PST) Date: Tue, 25 Nov 2025 14:21:16 -0500 From: Peter Xu To: Mike Rapoport Cc: linux-mm@kvack.org, Andrea Arcangeli , Andrew Morton , Axel Rasmussen , Baolin Wang , David Hildenbrand , Hugh Dickins , James Houghton , "Liam R. Howlett" , Lorenzo Stoakes , Michal Hocko , Nikita Kalyazin , Paolo Bonzini , Sean Christopherson , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, "David Hildenbrand (Red Hat)" Subject: Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason Message-ID: References: <20251125183840.2368510-1-rppt@kernel.org> <20251125183840.2368510-4-rppt@kernel.org> MIME-Version: 1.0 In-Reply-To: <20251125183840.2368510-4-rppt@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: RazzHxpGvOUxgrAjYhuFP1ibMiqtJ4Zye-QIRDlw_2w_1764098479 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: j5ji7htae3oeuktj8k95s7j3orgdfwbr X-Rspam-User: X-Rspamd-Queue-Id: 77AE28000F X-Rspamd-Server: rspam09 X-HE-Tag: 1764098483-539310 X-HE-Meta: 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 0gs2izmG 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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: Hi, Mike, On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote: > From: "Mike Rapoport (Microsoft)" > > When a VMA is registered with userfaulfd in minor mode, its ->fault() > method should check if a folio exists in the page cache and if yes > ->fault() should call handle_userfault(VM_UFFD_MISSING). s/MISSING/MINOR/ > > Instead of calling handle_userfault() directly from a specific ->fault() > implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will > notify the core page fault handler that it should call > handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault. Same. > > Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the Same. > new VM_FAULT_UFFD_MINOR there instead. Personally I'd keep the fault path as simple as possible, because that's the more frequently used path (rather than when userfaultfd is armed). I also see it slightly a pity that even with flags introduced, it only solves the MINOR problem, not MISSING. If it's me, I'd simply export handle_userfault().. I confess I still don't know why exporting it is a problem, but maybe I missed something. Only my two cents. Feel free to go with whatever way you prefer. Thanks, -- Peter Xu