linux-kernel - Re: [PATCH] arm64: configurable sparsemem section size

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Date:   Thu, 25 Apr 2019 16:31:38 +0100
From:   Will Deacon <will.deacon@....com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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.

Will