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@soleen.c...
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