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(p200300d82f250700d84615f36ca08029.dip0.t-ipconnect.de. [2003:d8:2f25:700:d846:15f3:6ca0:8029]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45de16b8b58sm99810995e9.4.2025.09.08.10.37.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 10:37:03 -0700 (PDT) Message-ID: <7b0f5b81-e18c-4629-a715-b5fee722b4aa@redhat.com> Date: Mon, 8 Sep 2025 19:36:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers To: Jason Gunthorpe Cc: Lorenzo Stoakes , Andrew Morton , Jonathan Corbet , Matthew Wilcox , Guo Ren , Thomas Bogendoerfer , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , "David S . 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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 References: <20250908133224.GE616306@nvidia.com> <090675bd-cb18-4148-967b-52cca452e07b@lucifer.local> <20250908142011.GK616306@nvidia.com> <764d413a-43a3-4be2-99c4-616cd8cd3998@lucifer.local> <20250908151637.GM616306@nvidia.com> <8edb13fc-e58d-4480-8c94-c321da0f4d8e@redhat.com> <20250908153342.GA789684@nvidia.com> <365c1ec2-cda6-4d94-895c-b2a795101857@redhat.com> <3229ac90-943f-4574-a9b8-bd4f5fa6cf03@redhat.com> <20250908155652.GE789684@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: B458418000D X-Stat-Signature: a4ec631panagg35y949s6trnp63uaph9 X-HE-Tag: 1757353030-432196 X-HE-Meta: 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 22ggSCF7 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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: On 08.09.25 17:56, Jason Gunthorpe wrote: > On Mon, Sep 08, 2025 at 05:50:18PM +0200, David Hildenbrand wrote: > >> So in practice there is indeed not a big difference between a private and >> cow mapping. > > Right and most drivers just check SHARED. > > But if we are being documentative why they check shared is because the > driver cannot tolerate COW. > > I think if someone is cargo culting a diver and sees > 'vma_never_cowable' they will have a better understanding of the > driver side issues. > > Driver's don't actually care about private vs shared, except this > indirectly implies something about cow. I recall some corner cases, but yes, most drivers don't clear MAP_MAYWRITE so is_cow_mapping() would just rule out what they wanted to rule out (no anon pages / cow semantics). FWIW, I recalled some VM_MAYWRITE magic in memfd, but it's really just for !cow mappings, so the following should likely work: diff --git a/mm/memfd.c b/mm/memfd.c index 1de610e9f2ea2..2a3aa26444bbb 100644 --- a/mm/memfd.c +++ b/mm/memfd.c @@ -346,14 +346,11 @@ static int check_write_seal(vm_flags_t *vm_flags_ptr) vm_flags_t vm_flags = *vm_flags_ptr; vm_flags_t mask = vm_flags & (VM_SHARED | VM_WRITE); - /* If a private mapping then writability is irrelevant. */ - if (!(mask & VM_SHARED)) + /* If a CoW mapping then writability is irrelevant. */ + if (is_cow_mapping(vm_flags)) return 0; - /* - * New PROT_WRITE and MAP_SHARED mmaps are not allowed when - * write seals are active. - */ + /* New PROT_WRITE mappings are not allowed when write-sealed. */ if (mask & VM_WRITE) return -EPERM; -- Cheers David / dhildenb