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(p200300cbc70763008418c653d01f3bd2.dip0.t-ipconnect.de. [2003:cb:c707:6300:8418:c653:d01f:3bd2]) by smtp.gmail.com with ESMTPSA id m11-20020adff38b000000b001ef879a5930sm890520wro.61.2022.03.08.23.37.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Mar 2022 23:37:28 -0800 (PST) Message-ID: <89ae59de-5b74-22b6-0076-c1a9a6fa62e7@redhat.com> Date: Wed, 9 Mar 2022 08:37:26 +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 From: David Hildenbrand 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> <461e4d2b-9aa2-50d4-2c78-3f7fb3f6a2f6@redhat.com> 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: <461e4d2b-9aa2-50d4-2c78-3f7fb3f6a2f6@redhat.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 09:06, David Hildenbrand wrote: > 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. > Just a slight clarification after stumbling over shared zeropage code in follow_page_pte(): we do seem to support pinning the shared zeropage at least on the GUP-slow path. While I haven't played with it, I assume we'd have to implement+trigger unsharing in case we'd want to take a R/O pin on the shared zeropage. Of course, similar to file-backed MAP_PRIVATE handling, this is out of the scope of this series ("This change implies that whenever user space wrote to a private mapping (IOW, we have an anonymous page mapped), that GUP pins will always remain consistent: reliable R/O GUP pins of anonymous pages."). -- Thanks, David / dhildenb