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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: <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> 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 al72hCVi CvZHe0tQJyCBC/INN/AN4TVK26/LCxNOOxvW3HLS14oJc4Csml1gEvUI3gVezhERzLY7b2qIK+eSc6USsyOLZQJHjCb6g0W2jDy7vjbEPsmH9VlIUUszfY/cyQL3Sym46fHhe 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 Mon, Sep 08, 2025 at 07:36:59PM +0200, David Hildenbrand wrote: > 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: I was invovled in these dark arts :) Since we gate the check_write_seal() function (which is the one that removes VM_MAYWRITE) on the mapping being shared, then obviously we can't remove VM_MAYWRITE in the first place. The only other way VM_MAYWRITE could be got rid of is if it already a MAP_SHARED or MAP_SHARED_VALIDATE mapping without write permission, and then it'd fail this check anyway. So I think the below patch is fine! > > 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 > Cheers, Lorenzo