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([2001:b07:6468:f312:61dd:85cb:23fc:fd54]) by smtp.gmail.com with ESMTPSA id t124sm27268833wmg.31.2020.10.13.00.27.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Oct 2020 00:27:14 -0700 (PDT) Subject: Re: [PATCH 04/35] dmem: let pat recognize dmem To: yulei.kernel@gmail.com, akpm@linux-foundation.org, naoya.horiguchi@nec.com, viro@zeniv.linux.org.uk Cc: linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, xiaoguangrong.eric@gmail.com, kernellwp@gmail.com, lihaiwei.kernel@gmail.com, Yulei Zhang , Xiao Guangrong References: <87e23dfbac6f4a68e61d91cddfdfe157163975c1.1602093760.git.yuleixzhang@tencent.com> From: Paolo Bonzini Message-ID: <72f4ddeb-157a-808a-2846-dd9961a9c269@redhat.com> Date: Tue, 13 Oct 2020 09:27:13 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <87e23dfbac6f4a68e61d91cddfdfe157163975c1.1602093760.git.yuleixzhang@tencent.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 08/10/20 09:53, yulei.kernel@gmail.com wrote: > From: Yulei Zhang > > x86 pat uses 'struct page' by only checking if it's system ram, > however it is not true if dmem is used, let's teach pat to > recognize this case if it is ram but it is !pfn_valid() > > We always use WB for dmem and any attempt to change this > behavior will be rejected and WARN_ON is triggered > > Signed-off-by: Xiao Guangrong > Signed-off-by: Yulei Zhang Hooks like these will make it very hard to merge this series. I like the idea of struct page-backed memory, but this is a lot of code and I wonder if it's worth adding all these complications. One can already use mem= to remove the "struct page" cost for most of the host memory, and manage the allocation of the remaining memory in userspace with /dev/mem. What is the advantage of doing this in the kernel? Paolo