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[73.158.171.241]) by smtp.gmail.com with ESMTPSA id c128sm12728998pfa.189.2021.06.01.00.08.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Jun 2021 00:08:51 -0700 (PDT) Subject: Re: Sealed memfd & no-fault mmap To: Linus Torvalds Cc: Hugh Dickins , Simon Ser , Peter Xu , "Kirill A. Shutemov" , Matthew Wilcox , Dan Williams , "Kirill A. Shutemov" , Will Deacon , Linux Kernel Mailing List , David Herrmann , "linux-mm@kvack.org" , Greg Kroah-Hartman , "tytso@mit.edu" References: <7718ec5b-0a9e-ffa6-16f2-bc0b6afbd9ab@gmail.com> <80c87e6b-6050-bf23-2185-ded408df4d0f@gmail.com> <36fc2485-11f1-5252-904d-f26b63a6cd58@gmail.com> From: Ming Lin Message-ID: <0464f8dd-d082-b246-83ff-609f0f48de59@gmail.com> Date: Tue, 1 Jun 2021 00:08:48 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/31/2021 11:24 PM, Linus Torvalds wrote: > On Mon, May 31, 2021 at 11:13 AM Ming Lin wrote: >> >> OK, I borrowed code from filemap_xip.c and implemented this behavior. > > I think that "unmap page" is too complicated and fragile. > > The only page that could possibly validly be unmapped is a stale zero > page, but that code in shmem_unmap_nofault_page() seems to try to > handle other cases too (ie that whole page_remove_rmap() - afaik a > zero page has no rmap). > > I get the feeling that the simpler thing to do is to just say "if you > use MAP_NOSIGBUS, and you access pages that don't have a backing > store, you will get zero pages, and they will NOT BE SYNCHRONIZED with > the backing store possibly later being updated". > > IOW, just document the fact that a MAP_NOSIGBUS mapping isn't coherent > wrt shmem contents that are expanded and filled in later. > > Don't try to "fix" it - because any user that uses MAP_NOSIGBUS had > better just accept that it's not compatible with expanding the shmem > backing store later. > > Keep it simple and stupid. Hmm? Simon, Is this "simple" solution good enough for Wayland compositor use case?