From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 25 Apr 2019 17:41:56 +0200 From: Michal Hocko Subject: Re: [PATCH] arm64: configurable sparsemem section size Message-ID: <20190425154156.GZ12751@dhcp22.suse.cz> References: <20190423203843.2898-1-pasha.tatashin@soleen.com> <20190425152550.GY12751@dhcp22.suse.cz> <20190425153138.GC25193@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190425153138.GC25193@fuggles.cambridge.arm.com> Sender: owner-linux-mm@kvack.org To: Will Deacon Cc: Pavel Tatashin , jmorris@namei.org, sashal@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, akpm@linux-foundation.org, dave.hansen@linux.intel.com, dan.j.williams@intel.com, keith.busch@intel.com, vishal.l.verma@intel.com, dave.jiang@intel.com, zwisler@kernel.org, thomas.lendacky@amd.com, ying.huang@intel.com, fengguang.wu@intel.com, bp@suse.de, bhelgaas@google.com, baiyaowei@cmss.chinamobile.com, tiwai@suse.de, jglisse@redhat.com, catalin.marinas@arm.com, rppt@linux.vnet.ibm.com, ard.biesheuvel@linaro.org, andrew.murray@arm.com, james.morse@arm.com, marc.zyngier@arm.com, sboyd@kernel.org, linux-arm-kernel@lists.infradead.org List-ID: On Thu 25-04-19 16:31:38, Will Deacon wrote: > On Thu, Apr 25, 2019 at 05:25:50PM +0200, Michal Hocko wrote: > > On Tue 23-04-19 16:38:43, Pavel Tatashin wrote: > > > sparsemem section size determines the maximum size and alignment that > > > is allowed to offline/online memory block. The bigger the size the less > > > the clutter in /sys/devices/system/memory/*. On the other hand, however, > > > there is less flexability in what granules of memory can be added and > > > removed. > > > > > > Recently, it was enabled in Linux to hotadd persistent memory that > > > can be either real NV device, or reserved from regular System RAM > > > and has identity of devdax. > > > > > > The problem is that because ARM64's section size is 1G, and devdax must > > > have 2M label section, the first 1G is always missed when device is > > > attached, because it is not 1G aligned. > > > > > > Allow, better flexibility by making section size configurable. > > > > Is there any inherent reason (64k page size?) that enforces such a large > > memsection? > > I gave *vague* memories of running out of bits in the page flags if we > changed this, but that was a while back. If that's no longer the case, > then I'm open to changing the value, but I really don't want to expose > it as a Kconfig option as proposed in this patch. People won't have a > clue what to set and it doesn't help at all with the single-Image effort. Ohh, I absolutely agree about the config option part JFTR. 1GB section loos quite excessive. I am not really sure a standard arm64 memory layout looks though. -- Michal Hocko SUSE Labs