On Wed 22-02-17 10:32:34, Vitaly Kuznetsov wrote:
[...]

quoted

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

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