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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pjtYI-00FzSa-1Y; Wed, 05 Apr 2023 03:11:14 +0000 Date: Wed, 5 Apr 2023 04:11:14 +0100 From: Matthew Wilcox To: Kyungsan Kim Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-cxl@vger.kernel.org, a.manzanares@samsung.com, viacheslav.dubeyko@bytedance.com, dan.j.williams@intel.com, seungjun.ha@samsung.com, wj28.lee@samsung.com Subject: Re: Re: RE: FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL Message-ID: References: <20230405020121.413658-1-ks0204.kim@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230405020121.413658-1-ks0204.kim@samsung.com> X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 8876E1C0011 X-Rspam-User: X-Stat-Signature: bkcxw9jgx45mhabjztadyk7wgajytaay X-HE-Tag: 1680664284-47335 X-HE-Meta: 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 AFZ1nguM pbvuj8sa9O9PS7Ll3NVR3vrIJJx67QWeCOEv6ShARZ1Y4p5sDHdjg2XLlJmOvJUwxgIhfJIWTYMX3fa9QzpRJLMTJ/BKmqmZam4PsyROnseq0fuKd7W/y5yg+xj6dIr/tkOH1Pxfkm9YLBUbPIIwZ4svPIXveL9OCrJFjItyTGek7lzDTRI56ZdJyvnuDhJgqYr9IGf6LxNi4u4lKafyMkqlwDg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Apr 05, 2023 at 11:01:21AM +0900, Kyungsan Kim wrote: > >1. Should we have the page allocator return pages from CXL or should > > CXL memory be allocated another way? > I think yes. Using CXL DRAM as System RAM interface would be the primary use case in real-world application in regards to compatibility. > So, on the System RAM interface, we think it should be managed by Linux MM subsystem. (Node - Zonelist - buddy page allocator) I don't think this is the right approach. > >2. Should there be a way for userspace to indicate that it prefers CXL > > memory when it calls mmap(), or should it always be at the discretion > > of the kernel? > I think yes. Both implcit and explict ways are meaningful for users on a different purpose. > The dynamic performance variation of CXL DRAM is likely bigger than other memory types due to the topology expansion and link negotiation. > I think it strengthens the needs. I also disagree with your answer here. > >3. Do we continue with the current ZONE_DEVICE model, or do we come up > > with something new? > In fact, ZONE_DEVICE was the our first candidate for CXL DRAM. > But because ZONE_DEVICE is not managed by buddy, we thought it does not fit to provide System RAM interface. But what you're proposing (separate GFP_EXMEM, ZONE_EXMEM, etc) doesn't let the buddy allocator satisfy GFP_KERNEL allocations from CXL. So what's the point?