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(p200300cbc7044900849bf76e5e1fff95.dip0.t-ipconnect.de. [2003:cb:c704:4900:849b:f76e:5e1f:ff95]) by smtp.gmail.com with ESMTPSA id i10-20020a0560001aca00b00203daf3759asm15006242wry.68.2022.03.21.09.24.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Mar 2022 09:24:07 -0700 (PDT) Message-ID: <77272de6-2c30-ea2a-c0e3-be89e47cf415@redhat.com> Date: Mon, 21 Mar 2022 17:24:06 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages Content-Language: en-US To: Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , 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: <20220315104741.63071-1-david@redhat.com> <20220315104741.63071-14-david@redhat.com> <20220318233035.GA11336@nvidia.com> <20220321161823.GZ11336@nvidia.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220321161823.GZ11336@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 21.03.22 17:18, Jason Gunthorpe wrote: > On Mon, Mar 21, 2022 at 05:15:06PM +0100, David Hildenbrand wrote: >> On 19.03.22 00:30, Jason Gunthorpe wrote: >>> On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote: >>>> Whenever GUP currently ends up taking a R/O pin on an anonymous page that >>>> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault >>>> on the page table entry will end up replacing the mapped anonymous page >>>> due to COW, resulting in the GUP pin no longer being consistent with the >>>> page actually mapped into the page table. >>>> >>>> The possible ways to deal with this situation are: >>>> (1) Ignore and pin -- what we do right now. >>>> (2) Fail to pin -- which would be rather surprising to callers and >>>> could break user space. >>>> (3) Trigger unsharing and pin the now exclusive page -- reliable R/O >>>> pins. >>>> >>>> We want to implement 3) because it provides the clearest semantics and >>>> allows for checking in unpin_user_pages() and friends for possible BUGs: >>>> when trying to unpin a page that's no longer exclusive, clearly >>>> something went very wrong and might result in memory corruptions that >>>> might be hard to debug. So we better have a nice way to spot such >>>> issues. >>>> >>>> To implement 3), we need a way for GUP to trigger unsharing: >>>> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped >>>> anonymous pages and resembles COW logic during a write fault. However, in >>>> contrast to a write fault, GUP-triggered unsharing will, for example, still >>>> maintain the write protection. >>> >>> Given the way this series has developed you might want to call this >>> FAULT_FLAG_MAKE_ANON_EXCLUSIVE >>> >>> Which strikes me as more directly connected to what it is trying to >>> do. >> >> I thought about something similar along those lines, and I think it >> would apply even when extending that mechanism to anything !anon inside >> a MAP_PRIVATE mapping. >> >> The whole >> >> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE; > > I think the extra words are worthwhile, share makes me think about > MAP_SHARED as we don't really use shared anywhere else FWICT.. Yeah, my point would be that you can only "unshare" in MAP_PRIVATE (!MAP_SHARED) :) -- Thanks, David / dhildenb