Re: memory hotplug and force_remove

From: Michal Hocko
Date: Tue Mar 28 2017 - 03:58:35 EST


On Mon 20-03-17 22:24:42, Rafael J. Wysocki wrote:
> On Monday, March 20, 2017 03:29:39 PM Michal Hocko wrote:
> > Hi Rafael,
>
> Hi,
>
> > we have been chasing the following BUG() triggering during the memory
> > hotremove (remove_memory):
> > ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> > check_memblock_offlined_cb);
> > if (ret)
> > BUG();
> >
> > and it took a while to learn that the issue is caused by
> > /sys/firmware/acpi/hotplug/force_remove being enabled. I was really
> > surprised to see such an option because at least for the memory hotplug
> > it cannot work at all. Memory hotplug fails when the memory is still
> > in use. Even if we do not BUG() here enforcing the hotplug operation
> > will lead to problematic behavior later like crash or a silent memory
> > corruption if the memory gets onlined back and reused by somebody else.
> >
> > I am wondering what was the motivation for introducing this behavior and
> > whether there is a way to disallow it for memory hotplug. Or maybe drop
> > it completely. What would break in such a case?
>
> Honestly, I don't remember from the top of my head and I haven't looked at
> that code for several months.
>
> I need some time to recall that.

Did you have any chance to look into this?
--
Michal Hocko
SUSE Labs