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Date: Thu, 25 Apr 2019 17:41:56 +0200 From: Michal Hocko <mhocko@...nel.org> To: Will Deacon <will.deacon@....com> Cc: Pavel Tatashin <pasha.tatashin@...een.com>, jmorris@...ei.org, sashal@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-nvdimm@...ts.01.org, akpm@...ux-foundation.org, dave.hansen@...ux.intel.com, dan.j.williams@...el.com, keith.busch@...el.com, vishal.l.verma@...el.com, dave.jiang@...el.com, zwisler@...nel.org, thomas.lendacky@....com, ying.huang@...el.com, fengguang.wu@...el.com, bp@...e.de, bhelgaas@...gle.com, baiyaowei@...s.chinamobile.com, tiwai@...e.de, jglisse@...hat.com, catalin.marinas@....com, rppt@...ux.vnet.ibm.com, ard.biesheuvel@...aro.org, andrew.murray@....com, james.morse@....com, marc.zyngier@....com, sboyd@...nel.org, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCH] arm64: configurable sparsemem section size 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
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