• Michael S. Tsirkin's avatar
    virtio_balloon: fix deadlock on OOM · c7cdff0e
    Michael S. Tsirkin authored
    fill_balloon doing memory allocations under balloon_lock
    can cause a deadlock when leak_balloon is called from
    virtballoon_oom_notify and tries to take same lock.
    
    To fix, split page allocation and enqueue and do allocations outside the lock.
    
    Here's a detailed analysis of the deadlock by Tetsuo Handa:
    
    In leak_balloon(), mutex_lock(&vb->balloon_lock) is called in order to
    serialize against fill_balloon(). But in fill_balloon(),
    alloc_page(GFP_HIGHUSER[_MOVABLE] | __GFP_NOMEMALLOC | __GFP_NORETRY) is
    called with vb->balloon_lock mutex held. Since GFP_HIGHUSER[_MOVABLE]
    implies __GFP_DIRECT_RECLAIM | __GFP_IO | __GFP_FS, despite __GFP_NORETRY
    is specified, this allocation attempt might indirectly depend on somebody
    else's __GFP_DIRECT_RECLAIM memory allocation. And such indirect
    __GFP_DIRECT_RECLAIM memory allocation might call leak_balloon() via
    virtballoon_oom_notify() via blocking_notifier_call_chain() callback via
    out_of_memory() when it reached __alloc_pages_may_oom() and held oom_lock
    mutex. Since vb->balloon_lock mutex is already held by fill_balloon(), it
    will cause OOM lockup.
    
      Thread1                                       Thread2
        fill_balloon()
          takes a balloon_lock
          balloon_page_enqueue()
            alloc_page(GFP_HIGHUSER_MOVABLE)
              direct reclaim (__GFP_FS context)       takes a fs lock
                waits for that fs lock                  alloc_page(GFP_NOFS)
                                                          __alloc_pages_may_oom()
                                                            takes the oom_lock
                                                            out_of_memory()
                                                              blocking_notifier_call_chain()
                                                                leak_balloon()
                                                                  tries to take that balloon_lock and deadlocks
    Reported-by: default avatarTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Wei Wang <wei.w.wang@intel.com>
    Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
    c7cdff0e