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

[Pavel Tatashin]

> > > 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);
>

Hi Micahl,

> 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?

I did.

> 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.

I was hitting this issue:
mm/memory_hotplug: drain per-cpu pages again during memory offline
https://lore.kernel.org/lkml/20200901124615.137200-1-pasha.tatashin@xxxxxxxxxx

Once the pcp drain  race is fixed, this particular deadlock becomes irrelavent.

The lock ordering, however, cgroup_mutex ->  mem_hotplug_lock is bad,
and the first race condition that I was hitting and described above is
still present. For now I added a temporary workaround by using save to
file instead of piping the core during shutdown. I am glad the
mainline is fixed, but stables should also have some kind of fix for
this problem.

Pasha