Re: [PATCH v2 00/28] The new cgroup slab memory controller

[Michal Hocko]

On Wed 02-09-20 11:53:00, Vlastimil Babka wrote:
> On 8/28/20 6:47 PM, Pavel Tatashin wrote:
> > There appears to be another problem that is related to the
> > cgroup_mutex -> mem_hotplug_lock deadlock described above.
> > 
> > In the original deadlock that I described, the workaround is to
> > replace crash dump from piping to Linux traditional save to files
> > method. However, after trying this workaround, I still observed
> > hardware watchdog resets during machine  shutdown.
> > 
> > The new problem occurs for the following reason: upon shutdown systemd
> > calls a service that hot-removes memory, and if hot-removing fails for
> 
> Why is that hotremove even needed if we're shutting down? Are there any
> (virtualization?) platforms where it makes some difference over plain
> shutdown/restart?

Yes this sounds quite dubious.

> > some reason systemd kills that service after timeout. However, systemd
> > is never able to kill the service, and we get hardware reset caused by
> > watchdog or a hang during shutdown:
> > 
> > Thread #1: memory hot-remove systemd service
> > Loops indefinitely, because if there is something still to be migrated
> > this loop never terminates. However, this loop can be terminated via
> > signal from systemd after timeout.
> > __offline_pages()
> >       do {
> >           pfn = scan_movable_pages(pfn, end_pfn);
> >                   # Returns 0, meaning there is nothing available to
> >                   # migrate, no page is PageLRU(page)
> >           ...
> >           ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> >                                             NULL, check_pages_isolated_cb);
> >                   # Returns -EBUSY, meaning there is at least one PFN that
> >                   # still has to be migrated.
> >       } while (ret);

This shouldn't really happen. What does prevent from this to proceed?
Did you manage to catch the specific pfn and what is it used for?
start_isolate_page_range and scan_movable_pages should fail if there is
any memory that cannot be migrated permanently. This is something that
we should focus on when debugging.
-- 
Michal Hocko
SUSE Labs