Re: [RFC PATCH] memory-hotplug: Use dev_online for memhp_auto_offline

[Michal Hocko <mhocko@kernel.org>]

On Wed 22-02-17 10:32:34, Vitaly Kuznetsov wrote:
[...]
> > There is a workaround in that a user could online the memory or have
> > a udev rule to online the memory by using the sysfs interface. The
> > sysfs interface to online memory goes through device_online() which
> > should updated the dev->offline flag. I'm not sure that having kernel
> > memory hotplug rely on userspace actions is the correct way to go.
> 
> Using udev rule for memory onlining is possible when you disable
> memhp_auto_online but in some cases it doesn't work well, e.g. when we
> use memory hotplug to address memory pressure the loop through userspace
> is really slow and memory consuming, we may hit OOM before we manage to
> online newly added memory.

How does the in-kernel implementation prevents from that?

> In addition to that, systemd/udev folks
> continuosly refused to add this udev rule to udev calling it stupid as
> it actually is an unconditional and redundant ping-pong between kernel
> and udev.

This is a policy and as such it doesn't belong to the kernel. The whole
auto-enable in the kernel is just plain wrong IMHO and we shouldn't have
merged it.
-- 
Michal Hocko
SUSE Labs