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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: References: <20250908125101.GX616306@nvidia.com> <20250908133224.GE616306@nvidia.com> <090675bd-cb18-4148-967b-52cca452e07b@lucifer.local> <20250908142011.GK616306@nvidia.com> <764d413a-43a3-4be2-99c4-616cd8cd3998@lucifer.local> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: LO0P123CA0013.GBRP123.PROD.OUTLOOK.COM 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2025 at 05:07:57PM +0200, David Hildenbrand wrote: > On 08.09.25 16:47, Lorenzo Stoakes wrote: > > 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? > > I'll note that the naming is bad. > > Why? > > Because the vma_desc is not cowable. The underlying mapping maybe is. Right, but the vma_desc desribes a VMA being set up. I mean is_cow_mapping(desc->vm_flags) isn't too egregious anyway, so maybe just use that for that case? > > -- > Cheers > > David / dhildenb > Cheers, Lorenzo