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

[Masayoshi Mizuma]

On Wed, Sep 19, 2018 at 02:48:06PM +0200, Ingo Molnar wrote:
> 
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> > On Wed, 19 Sep 2018, Ingo Molnar wrote:
> > > * Masayoshi Mizuma <msys.mizuma@xxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > > > 
> > > > > 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.
> > 
> > As I understand it, it's depending on the actual physical characteristics
> > of the machine. So setting a fixed value in Kconfig might work for one, but
> > not for others and having a command line option allows to tweak that at
> > boot time and having a common kernel image.
> > 
> > Ideally we would calculate that from SRAT, but AFAICT SRAT is not available
> > at the point where this needs to be done.

Yes, that's right. The KASLR initialization is early boot sequence,
so SRAT is not available at that time.

> 
> Yeah, so could we at least do something like this:
> 
>  - See whether using the maximum padding as the new default padding would work for everyone?
>    A bit more virtual memory used, or are there other costs as well?

The current default padding size if CONFIG_MEMORY_HOTPLUG set is 10TB.
IMO, it should not be increased because it gets the available entropy
decreased...

>  - Add checking code to the later SRAT case to at least _detect_ bad padding after the fact.
>    We don't utilize RAM with bad padding until that, right?

I have an idea as following. Does that make sense?

Add a warning message which shows the padding size is not enough
for the physical memory mapping and tell to the user about
recommended padding size. User can change the padding size in next
reboot to add the boot parameter.

> 
>  - Add 'quirk' to the name of the boot parameter, to make it clear that this is really due to 
>    suboptimal communication between the firmware and the kernel.

I'm ok if 'quirk' is added to the boot parameter.

Thanks,
Masa