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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id x81sm81282wmb.11.2020.10.19.06.37.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Oct 2020 06:37:52 -0700 (PDT) Subject: Re: [PATCH 00/35] Enhance memory utilization with DMEMFS To: Dan Williams , Joao Martins Cc: yulei zhang , linux-fsdevel , kvm , LKML , Xiao Guangrong , Wanpeng Li , Haiwei Li , Yulei Zhang , Andrew Morton , Naoya Horiguchi , Al Viro , Matthew Wilcox , Mike Kravetz , Jane Y Chu , Muchun Song , Konrad Rzeszutek Wilk References: <98be093d-c869-941a-6dd9-fb16356f763b@oracle.com> From: Paolo Bonzini Message-ID: <3626f5ff-b6a0-0811-5899-703a0714897d@redhat.com> Date: Mon, 19 Oct 2020 15:37:51 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/10/20 00:25, Dan Williams wrote: > Now, with recent device-dax extensions, it > also has a coarse grained memory management system for physical > address-space partitioning and a path for struct-page-less backing for > VMs. What feature gaps remain vs dmemfs, and can those gaps be closed > with incremental improvements to the 2 existing memory-management > systems? If I understand correctly, devm_memremap_pages() on ZONE_DEVICE memory would still create the "struct page" albeit lazily? KVM then would use the usual get_user_pages() path. Looking more closely at the implementation of dmemfs, I don't understand is why dmemfs needs VM_DMEM etc. and cannot provide access to mmap-ed memory using remap_pfn_range and VM_PFNMAP, just like /dev/mem. If it did that KVM would get physical addresses using fixup_user_fault and never need pfn_to_page() or get_user_pages(). I'm not saying that would instantly be an approval, but it would make remove a lot of hooks. Paolo