linux-kernel - Re: [PATCH] mm: don't rely on system state to detect hot-plug operations

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Message-ID: <20200910133854.GA8713@linux>
Date:   Thu, 10 Sep 2020 15:39:00 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>, akpm@...ux-foundation.org,
        David Hildenbrand <david@...hat.com>, rafael@...nel.org,
        nathanl@...ux.ibm.com, cheloha@...ux.ibm.com,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations

On Thu, Sep 10, 2020 at 02:48:47PM +0200, Michal Hocko wrote:
> > Is there any actual usecase for a configuration like this? What is the
> > point to statically define additional memory like this when the same can
> > be achieved on the same command line?

Well, for qemu I am not sure, but if David is right, it seems you can face
the same if you reboot a vm with hotplugged memory.
Moreover, it seems that the problem we spotted with [1], it was a VM running on
Promox (KVM).
The Hypervisor probably said at boot time "Ey, I do have these ACPI devices, care
to enable them now"?

As always, there are all sorts of configurations/scenarios out there in the wild.

> Forgot to ask one more thing. Who is going to online that memory when
> userspace is not running yet?

Depends, if you have CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE set or you specify
memhp_default_online_type=[online,online_*], memory will get onlined right
after hot-adding stage:

        /* online pages if requested */
        if (memhp_default_online_type != MMOP_OFFLINE)
                walk_memory_blocks(start, size, NULL, online_memory_block);

If not, systemd-udev will do the magic once the system is up.

-- 
Oscar Salvador
SUSE L3