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Andrea Arcangeli authored
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 FOLL_NOWAIT. Link: http://lkml.kernel.org/r/20180302174343.5421-2-aarcange@redhat.com Fixes: ce53053c ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()") Signed-off-by:
Andrea Arcangeli <aarcange@redhat.com> Reported-by:
Dr. David Alan Gilbert <dgilbert@redhat.com> Tested-by:
Dr. David Alan Gilbert <dgilbert@redhat.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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