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

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Message-ID: <20200910135106.GI28354@dhcp22.suse.cz>
Date:   Thu, 10 Sep 2020 15:51:06 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
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 10-09-20 15:39:00, Oscar Salvador wrote:
> 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.

OK, thanks for the clarification. I was not aware of the reboot.

> 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.

Does that imply that we need udev to scan all existing devices and
reprobe them?
-- 
Michal Hocko
SUSE Labs