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

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Message-ID: <20190425152550.GY12751@dhcp22.suse.cz>
Date:   Thu, 25 Apr 2019 17:25:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     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, will.deacon@....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 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?
-- 
Michal Hocko
SUSE Labs