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(p200300cbc7063600f39ad830baab42f0.dip0.t-ipconnect.de. [2003:cb:c706:3600:f39a:d830:baab:42f0]) by smtp.gmail.com with ESMTPSA id v12-20020a5d4a4c000000b001e68ba61747sm1293716wrs.16.2022.03.03.00.06.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Mar 2022 00:06:03 -0800 (PST) Message-ID: <461e4d2b-9aa2-50d4-2c78-3f7fb3f6a2f6@redhat.com> Date: Thu, 3 Mar 2022 09:06:00 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Content-Language: en-US To: John Hubbard , Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Vlastimil Babka , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Liang Zhang , Pedro Gomes , Oded Gabbay , linux-mm@kvack.org References: <20220224122614.94921-1-david@redhat.com> <20220224122614.94921-13-david@redhat.com> <20220302165559.GU219866@nvidia.com> <0a159b65-cb80-c8eb-7ad1-24b83813531f@nvidia.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page In-Reply-To: <0a159b65-cb80-c8eb-7ad1-24b83813531f@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03.03.22 02:47, John Hubbard wrote: > On 3/2/22 12:38, David Hildenbrand wrote: > ... >> BUT, once we actually write to the private mapping via the page table, >> the GUP pin would go out of sync with the now-anonymous page mapped into >> the page table. However, I'm having a hard time answering what's >> actually expected? >> >> It's really hard to tell what the user wants with MAP_PRIVATE file >> mappings and stumbles over a !anon page (no modifications so far): >> >> (a) I want a R/O pin to observe file modifications. >> (b) I want the R/O pin to *not* observe file modifications but observe >> my (eventual? if any) private modifications, >> > > On this aspect, I think it is easier than trying to discern user > intentions. Because it is less a question of what the user wants, and > more a question of how mmap(2) is specified. And the man page clearly > indicates that the user has no right to expect to see file > modifications. Here's the excerpt: > > "MAP_PRIVATE > > Create a private copy-on-write mapping. Updates to the mapping are not > visible to other processes mapping the same file, and are not carried > through to the underlying file. It is unspecified whether changes made > to the file after the mmap() call are visible in the mapped region. > " > >> Of course, if we already wrote to that page and now have an anon page, >> it's easy: we are already no longer following file changes. > > Yes, and in fact, I've always thought that the way this was written > means that it should be treated as a snapshot of the file contents, > and no longer reliably connected in either direction to the page(s). Thanks John, that's extremely helpful. I forgot about these MAP_PRIVATE mmap() details -- they help a lot to clarify which semantics to provide. So what we could do is: a) Extend FAULT_FLAG_UNSHARE to also unshare an !anon page in a MAP_RPIVATE mapping, replacing it with an (exclusive) anon page. R/O PTE permissions are maintained, just like unsharing in the context of this series. b) Similarly trigger FAULT_FLAG_UNSHARE from GUP when trying to take a R/O pin (FOLL_PIN) on a R/O-mapped !anon page in a MAP_PRIVATE mapping. c) Make R/O pins consistently use "FOLL_PIN" instead, getting rid of FOLL_FORCE|FOLL_WRITE. Of course, we can't detect MAP_PRIVATE vs. MAP_SHARED in GUP-fast (no VMA), so we'd always have to fallback in GUP-fast in case we intend to FOLL_PIN a R/O-mapped !anon page. That would imply that essentially any R/O pins (FOLL_PIN) would have to fallback to ordinary GUP. BUT, I mean we require FOLL_FORCE|FOLL_WRITE right now, which is not any different, so ... One optimization would be to trigger b) only for FOLL_LONGTERM. For !FOLL_LONGTERM there are "in theory" absolutely no guarantees which data will be observed if we modify concurrently to e.g., O_DIRECT IMHO. But that would require some more thought. Of course, that's all material for another journey, although it should be mostly straight forward. -- Thanks, David / dhildenb