virtio_balloon: fix race by fill and leak
commit f68b992b upstream. During my compaction-related stuff, I encountered a bug with ballooning. With repeated inflating and deflating cycle, guest memory( ie, cat /proc/meminfo | grep MemTotal) is decreased and couldn't be recovered. The reason is balloon_lock doesn't cover release_pages_balloon so struct virtio_balloon fields could be overwritten by race of fill_balloon(e,g, vb->*pfns could be critical). This patch fixes it in my test. Signed-off-by: Minchan Kim <minchan@kernel.org> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> [ luis: backported to 3.16: adjusted context ] Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
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