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Subject: Re: Memory hotplug softlock issue
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Baoquan He <bhe@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        aarcange@redhat.com
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From:   David Hildenbrand <david@redhat.com>
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Date:   Wed, 14 Nov 2018 10:22:31 +0100
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>>
>> 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@kernel.org
> 


-- 

Thanks,

David / dhildenb