On Fri,  2 Mar 2018 18:43:43 +0100 Andrea Arcangeli <aarca...@redhat.com> wrote:

> KVM is hanging during postcopy live migration with userfaultfd because
> get_user_pages_unlocked is not capable to handle FOLL_NOWAIT.
> Earlier FOLL_NOWAIT was only ever passed to get_user_pages.
> Specifically faultin_page (the callee of get_user_pages_unlocked
> caller) doesn't know that if FAULT_FLAG_RETRY_NOWAIT was set in the
> page fault flags, when VM_FAULT_RETRY is returned, the mmap_sem wasn't
> actually released (even if nonblocking is not NULL). So it sets
> *nonblocking to zero and the caller won't release the mmap_sem
> thinking it was already released, but it wasn't because of
> Reported-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Tested-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Signed-off-by: Andrea Arcangeli <aarca...@redhat.com>

I added

Fixes: ce53053ce378c ("kvm: switch get_user_page_nowait() to 
Cc: <sta...@vger.kernel.org>

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