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[73.158.171.241]) by smtp.gmail.com with ESMTPSA id bv3sm501772pjb.1.2021.05.31.14.13.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 31 May 2021 14:13:34 -0700 (PDT) Subject: Re: Sealed memfd & no-fault mmap From: Ming Lin To: Hugh Dickins , Linus Torvalds , Simon Ser Cc: 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: <20210429154807.hptls4vnmq2svuea@box> <20210429183836.GF8339@xz-x1> <7718ec5b-0a9e-ffa6-16f2-bc0b6afbd9ab@gmail.com> <80c87e6b-6050-bf23-2185-ded408df4d0f@gmail.com> <36fc2485-11f1-5252-904d-f26b63a6cd58@gmail.com> Message-ID: Date: Mon, 31 May 2021 14:13:28 -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: <36fc2485-11f1-5252-904d-f26b63a6cd58@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/29/2021 4:36 PM, Ming Lin wrote: > On 5/29/2021 1:15 PM, Hugh Dickins wrote: > >> >> I believe the correct behaviour would be to unmap the nofault page >> then, allowing the proper page to be faulted in after.  That is >> certainly doable (the old mm/filemap_xip.c used to do so), but might >> get into some awkward race territory, with filesystem dependence >> (reminiscent of hole punch, in reverse).  shmem could operate that >> way, and be the better for it: but I wouldn't want to add that, >> without also cleaning away all the shmem_recalc_inode() stuff. OK, I borrowed code from filemap_xip.c and implemented this behavior. Simon, Before I send out the patches for review, would you mind have a quick test? https://github.com/minggr/linux, branch shmem_no_sigbus In Wayland compositors, you only need to pass in MAP_NOSIGBUS in mmap(). For example, //fd should be received from Wayland compositors client #define MAP_NOSIGBUS 0x200000 addr = mmap(NULL, size, PROT_READ, MAP_SHARED|MAP_NOSIGBUS, fd, offset) Thanks, Ming