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

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Message-ID: <CAOSf1CEihBDDRny5S_7TA5Cqbdgsh6zN=kdq83OSgaLbRphtrg@mail.gmail.com>
Date:   Fri, 3 Apr 2020 09:46:44 +1100
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Leonardo Bras <leonardo@...ux.ibm.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, Apr 3, 2020 at 6:55 AM Leonardo Bras <leonardo@...ux.ibm.com> wrote:
>
> While providing guests, it's desirable to resize it's memory on demand.
>
> By now, it's possible to do so by creating a guest with a small base
> memory, hot-plugging all the rest, and using 'movable_node' kernel
> command-line parameter, which puts all hot-plugged memory in
> ZONE_MOVABLE, allowing it to be removed whenever needed.
>
> But there is an issue regarding guest reboot:
> If memory is hot-plugged, and then the guest is rebooted, all hot-plugged
> memory goes to ZONE_NORMAL, which offers no guaranteed hot-removal.
> It usually prevents this memory to be hot-removed from the guest.
>
> It's possible to use device-tree information to fix that behavior, as
> it stores flags for LMB ranges on ibm,dynamic-memory-vN.
> It involves marking each memblock with the correct flags as hotpluggable
> memory, which mm/memblock.c puts in ZONE_MOVABLE during boot if
> 'movable_node' is passed.

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.