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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id n13sm3612685wmd.21.2020.02.25.02.03.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Feb 2020 02:03:52 -0800 (PST) Date: Tue, 25 Feb 2020 11:03:52 +0100 From: Michal Hocko To: Baoquan He Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, richardw.yang@linux.intel.com, david@redhat.com, osalvador@suse.de, dan.j.williams@intel.com, rppt@linux.ibm.com, robin.murphy@arm.com Subject: Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP case Message-ID: <20200225100352.GN22443@dhcp22.suse.cz> References: <20200220043316.19668-1-bhe@redhat.com> <20200220103849.GG20509@dhcp22.suse.cz> <20200221142847.GG4937@MiWiFi-R3L-srv> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200221142847.GG4937@MiWiFi-R3L-srv> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 21-02-20 22:28:47, Baoquan He wrote: > On 02/20/20 at 11:38am, Michal Hocko wrote: > > On Thu 20-02-20 12:33:09, Baoquan He wrote: > > > Memory sub-section hotplug was added to fix the issue that nvdimm could > > > be mapped at non-section aligned starting address. A subsection map is > > > added into struct mem_section_usage to implement it. However, sub-section > > > is only supported in VMEMMAP case. > > > > Why? Is there any fundamental reason or just a lack of implementation? > > VMEMMAP should be really only an implementation detail unless I am > > missing something subtle. > > Thanks for checking. > > VMEMMAP is one of two ways to convert a PFN to the corresponding > 'struct page' in SPARSE model. I mentioned them as VMEMMAP case, or > !VMEMMAP case because we called them like this previously when reviewed > patches, hope it won't cause confusion. > > Currently, config ZONE_DEVICE depends on SPARSEMEM_VMEMMAP. The > subsection_map is added to struct mem_section_usage to track which sub > section is present, VMEMMAP fills those bits which corresponding > sub-sections are present, while !VMEMMAP, namely classic SPARSE, fills > the whole map always. > > As we know, VMEMMAP builds page table to map a cluster of 'struct page' > into the corresponding area of 'vmemmap'. Subsection hotplug can be > supported naturally, w/o any change, just map needed region related to > sub-sections on demand. For !VMEMMAP, it allocates memmap with > alloc_pages() or vmalloc, thing is a little complicated, e.g the mixed > section, boot memory occupies the starting area, later pmem hot added to > the rear part. > > About !VMEMMAP which doesn't support sub-section hotplog, Dan said > it's more because the effort and maintenance burden outweighs the > benefit. And the current 64 bit ARCHes all enable > SPARSEMEM_VMEMMAP_ENABLE by default. OK, if this is the primary argument then make sure to document it in the changelog (cover letter). -- Michal Hocko SUSE Labs