>> >> The real question is, however, why offlining of the last block doesn't >> succeed. In __offline_pages() we basically have an endless loop (while >> holding the mem_hotplug_lock in write). Now I consider this piece of >> code very problematic (we should automatically fail after X >> attempts/after X seconds, we should not ignore -ENOMEM), and we've had >> other BUGs whereby we would run into an endless loop here (e.g. related >> to hugepages I guess). > > We used to have number of retries previous and it was too fragile. If > you need a timeout then you can easily do that from userspace. Just do > timeout $TIME echo 0 > $MEM_PATH/online I agree that number of retries is not a good measure. But as far as I can see this happens from the kernel via an ACPI event. E.g. failing to offline a block after X seconds would still make sense. (if something takes 120seconds to offline 128MB/2G there is something very bad going on, we could set the default limit to e.g. 30seconds), however ... > > I have seen an issue when the migration cannot make a forward progress > because of a glibc page with a reference count bumping up and down. Most > probable explanation is the faultaround code. I am working on this and > will post a patch soon. In any case the migration should converge and if > it doesn't do then there is a bug lurking somewhere. ... I also agree that this should converge. And if we detect a serious issue that we can't handle/where we can't converge (e.g. -ENOMEM) we should abort. > > Failing on ENOMEM is a questionable thing. I haven't seen that happening > wildly but if it is a case then I wouldn't be opposed. > >> You mentioned memory pressure, if our host is under memory pressure we >> can easily trigger running into an endless loop there, because we >> basically ignore -ENOMEM e.g. when we cannot get a page to migrate some >> memory to be offlined. I assume this is the case here. >> do_migrate_range() could be the bad boy if it keeps failing forever and >> we keep retrying. I've seen quite some issues while playing with virtio-mem, but didn't have the time to look into the details. Still on my long list of things to look into. > > My hotplug debugging patches [1] should help to tell us. > > [1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@xxxxxxxxxx > -- Thanks, David / dhildenb
