linux-kernel - Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests

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Date:   Thu, 02 Apr 2020 20:41:05 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     "Oliver O'Halloran" <oohall@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Allison Randal <allison@...utok.net>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after
 reboot on pseries guests

On Fri, 2020-04-03 at 10:31 +1100, Oliver O'Halloran wrote:
> On Fri, Apr 3, 2020 at 10:07 AM Leonardo Bras <leonardo@...ux.ibm.com> wrote:
> > Hello Oliver, thank you for the feedback.
> > Comments inline:
> > 
> > On Fri, 2020-04-03 at 09:46 +1100, Oliver O'Halloran wrote:
> > > I don't really understand why the flag is needed at all. According to
> > > PAPR any memory provided by dynamic reconfiguration can be hot-removed
> > > so why aren't we treating all DR memory as hot removable? The only
> > > memory guaranteed to be there 100% of the time is what's in the
> > > /memory@0 node since that's supposed to cover the real mode area.
> > 
> > All LMBs are listed in DR memory, even the base memory.
> > 
> > The v1 of the patch would work this way, as qemu would configure it's
> > DR memory with (DRC_INVALID | RESERVED) flags and the hot-added memory
> > with (ASSIGNED) flag. Looking for assigned flag would be enough.
> > 
> > But as of today, PowerVM doesn't seem to work that way.
> > When you boot a PowerVM virtual machine with Linux, all memory is added
> > with the same flags (ASSIGNED).
> > 
> > To create a solution that doesn't break PowerVM, this new flag was made
> > necessary.
> 
> I'm still not convinced it's necessary. Why not check memory@0 and use
> the size as a clip level? Any memory above that level gets marked as
> hotpluggable and anything below doesn't. Seems to me that would work
> on all current platforms, so what am I missing here?
> 

Humm, wouldn't that assume that all unmovable memory should be at the
first LMBs? 

If we use the recently approved flag, there would be no such
limitation, and there would be possible to do some other things, like
hot-adding more unmovable memory to kernel usage.

What do you think?

Best regards,

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