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Mon, 24 Apr 2023 09:40:29 -0400 (EDT) Received: by box.shutemov.name (Postfix, from userid 1000) id E05E310A0FA; Mon, 24 Apr 2023 16:40:26 +0300 (+03) Date: Mon, 24 Apr 2023 16:40:26 +0300 From: "Kirill A. Shutemov" To: Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default Message-ID: <20230424134026.di6nf2an3a2g63a6@box.shutemov.name> References: <20230424120247.k7cjmncmov32yv5r@box.shutemov.name> <3273f5f3-65d9-4366-9424-c688264992f9@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3273f5f3-65d9-4366-9424-c688264992f9@lucifer.local> X-Rspamd-Queue-Id: ECE6718000A X-Stat-Signature: 5rp156aomg7gdn3sgokihdoeenyb9yuz X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1682343631-800505 X-HE-Meta: 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 /BQq8s3w snenvk8ezEgZKUeO88sxhzl4Qn3NBEOuHH+SGK+GuASQEJgdMP408MKLx65Xviom3yB0a2y7VREK3w4ksCJmuIFpDrjlNkxvUGqTJM4/hIMLl3RmJat6RAFjkjxSjJRjBNJOR48VdILgAeAvSdLGN0svfDBzlibMaNevuIqV5dK2A0DxCOuuxm2/sQimP4lGtTDK8kMDAUePpEM/X/Mdzyo+mSbzNRc1Um3Nd5AM8Yd3tdy96YtL2puoQF+Jvpo3gscjid8bCIKx0Z9tbtTA50N149ufDlUgt5Y9/sNNfsyl3IV0i/9onmG9FiDba0R3l0rmARpOyKf/ypE6ZRBWvRZLfMEi65VsHlt6CMQML1rBQpRo= 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: On Mon, Apr 24, 2023 at 01:31:56PM +0100, Lorenzo Stoakes wrote: > On Mon, Apr 24, 2023 at 03:02:47PM +0300, Kirill A. Shutemov wrote: > > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote: > > > @@ -959,16 +959,46 @@ static int faultin_page(struct vm_area_struct *vma, > > > return 0; > > > } > > > > > > +/* > > > + * Writing to file-backed mappings using GUP is a fundamentally broken operation > > > + * as kernel write access to GUP mappings may not adhere to the semantics > > > + * expected by a file system. > > > + * > > > + * In most instances we disallow this broken behaviour, however there are some > > > + * exceptions to this enforced here. > > > + */ > > > +static inline bool can_write_file_mapping(struct vm_area_struct *vma, > > > + unsigned long gup_flags) > > > +{ > > > + struct file *file = vma->vm_file; > > > + > > > + /* If we aren't pinning then no problematic write can occur. */ > > > + if (!(gup_flags & (FOLL_GET | FOLL_PIN))) > > > + return true; > > > + > > > + /* Special mappings should pose no problem. */ > > > + if (!file) > > > + return true; > > > + > > > + /* Has the caller explicitly indicated this case is acceptable? */ > > > + if (gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING) > > > + return true; > > > + > > > + /* shmem and hugetlb mappings do not have problematic semantics. */ > > > + return vma_is_shmem(vma) || is_file_hugepages(file); > > > > Can this be generalized to any fs that doesn't have vm_ops->page_mkwrite()? > > > > Something more general would be preferable, however I believe there were > concerns broader than write notify, for instance not correctly marking the > folio dirty after writing to it, though arguably the caller should > certainly be ensuring that (and in many cases, do). It doesn't make much sense to me. Shared writable mapping without page_mkwrite (or pfn_write) will setup writeable PTE even on read faults[1], so you will not get the page dirty, unless you scan page table entries for dirty bit. [1] See logic around vm_page_prot vs. vma_wants_writenotify(). -- Kiryl Shutsemau / Kirill A. Shutemov