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Miller" , Andreas Larsson , Arnd Bergmann , Greg Kroah-Hartman , Dan Williams , Vishal Verma , Dave Jiang , Nicolas Pitre , Muchun Song , Oscar Salvador , David Hildenbrand , Konstantin Komarov , Baoquan He , Vivek Goyal , Dave Young , Tony Luck , Reinette Chatre , Dave Martin , James Morse , Alexander Viro , Christian Brauner , Jan Kara , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , Uladzislau Rezki , Dmitry Vyukov , Andrey Konovalov , Jann Horn , Pedro Falcato , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-csky@vger.kernel.org, linux-mips@vger.kernel.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-mm@kvack.org, ntfs3@lists.linux.dev, kexec@lists.infradead.org, kasan-dev@googlegroups.com Subject: Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers Message-ID: <764d413a-43a3-4be2-99c4-616cd8cd3998@lucifer.local> References: <20250908125101.GX616306@nvidia.com> <20250908133224.GE616306@nvidia.com> <090675bd-cb18-4148-967b-52cca452e07b@lucifer.local> <20250908142011.GK616306@nvidia.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250908142011.GK616306@nvidia.com> X-ClientProxiedBy: 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List-Unsubscribe: On Mon, Sep 08, 2025 at 11:20:11AM -0300, Jason Gunthorpe wrote: > On Mon, Sep 08, 2025 at 03:09:43PM +0100, Lorenzo Stoakes wrote: > > > Perhaps > > > > > > !vma_desc_cowable() > > > > > > Is what many drivers are really trying to assert. > > > > Well no, because: > > > > static inline bool is_cow_mapping(vm_flags_t flags) > > { > > return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE; > > } > > > > Read-only means !CoW. > > What drivers want when they check SHARED is to prevent COW. It is COW > that causes problems for whatever the driver is doing, so calling the > helper cowable and making the test actually right for is a good thing. > > COW of this VMA, and no possibilty to remap/mprotect/fork/etc it into > something that is COW in future. But you can't do that if !VM_MAYWRITE. I mean probably the driver's just wrong and should use is_cow_mapping() tbh. > > Drivers have commonly various things with VM_SHARED to establish !COW, > but if that isn't actually right then lets fix it to be clear and > correct. I think we need to be cautious of scope here :) I don't want to accidentally break things this way. OK I think a sensible way forward - How about I add desc_is_cowable() or vma_desc_cowable() and only set this if I'm confident it's correct? That way I can achieve both aims at once. > > Jason Cheers, Lorenzo