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    SubjectRe: Memory hotplug softlock issue
    On Wed 14-11-18 09:18:09, David Hildenbrand wrote:
    > On 14.11.18 08:09, Baoquan He wrote:
    > > Hi,
    > >
    > > Tested memory hotplug on a bare metal system, hot removing always
    > > trigger a lock. Usually need hot plug/unplug several times, then the hot
    > > removing will hang there at the last block. Surely with memory pressure
    > > added by executing "stress -m 200".
    > >
    > > Will attach the log partly. Any idea or suggestion, appreciated.
    > >
    > > Thanks
    > > Baoquan
    > >
    >
    > Code seems to be waiting for the mem_hotplug_lock in read.
    > We hold mem_hotplug_lock in write whenever we online/offline/add/remove
    > memory. There are two ways to trigger offlining of memory:
    >
    > 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state"
    >
    > This always properly took the mem_hotplug_lock. Nothing changed
    >
    > 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online"
    >
    > This didn't take the mem_hotplug_lock and I fixed that for this release.

    This discrepancy should go.

    > So if you were testing with 1., you should have seen the same error
    > before this release (unless there is something else now broken in this
    > release).
    >
    >
    > 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 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.

    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.

    My hotplug debugging patches [1] should help to tell us.

    [1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@kernel.org
    --
    Michal Hocko
    SUSE Labs

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