linux-kernel - Re: [PATCH v3 1/2] x86/mm: Add an option to change the padding used for the physical memory mapping

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Date:   Wed, 19 Sep 2018 14:17:20 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masayoshi Mizuma <msys.mizuma@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Baoquan He <bhe@...hat.com>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-kernel@...r.kernel.org, mike.travis@....com, sivanich@....com
Subject: Re: [PATCH v3 1/2] x86/mm: Add an option to change the padding used
 for the physical memory mapping


* Masayoshi Mizuma <msys.mizuma@...il.com> wrote:

> Ping...
> I would appreciate if someone could review it because this patch
> fixes the real memory hotplug issue...

Yeah, so I generally try to resist random new boot options that
work around real bugs, so please convince me that this patch
is the best option:

> 
> On Tue, Sep 04, 2018 at 11:11:40AM -0400, Masayoshi Mizuma wrote:
> > From: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> > 
> > If each node of physical memory layout has huge space for hotplug,
> > the padding used for the physical memory mapping section is not enough.
> > For exapmle of the layout:
> >   SRAT: Node 6 PXM 4 [mem 0x100000000000-0x13ffffffffff] hotplug
> >   SRAT: Node 7 PXM 5 [mem 0x140000000000-0x17ffffffffff] hotplug
> >   SRAT: Node 2 PXM 6 [mem 0x180000000000-0x1bffffffffff] hotplug
> >   SRAT: Node 3 PXM 7 [mem 0x1c0000000000-0x1fffffffffff] hotplug
> > 
> > We can increase the padding by CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING,
> > however, the needed padding size depends on the system environment.
> > The kernel option is better than changing the config.
> > 
> > Change log from v2:
> >  - Simplify the description. As Baoquan said, this is simillar SGI UV issue,
> >    but a little different. Remove SGI UV description.

Could you please explain it a bit better where the higher padding requirement comes from?

'system environment' is very opaque.

Thanks,

	Ingo