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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> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20251125183840.2368510-4-rppt@kernel.org> 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