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In other words, deny multiple read-only mounts of the same device. Shared read-only mounts should theoretically be possible, but, unfortunately, can not be implemented correctly using current buffer cache code/interface and results in an eventual system crash. Also, using nullfs seems to be a more efficient way to achieve the same goal. This gets us back to where we were before GEOM and where other BSDs are. Submitted by: pjd (idea for checking for shared mounting) Discussed with: phk, pjd Silence from: fs@, geom@ MFC after: 2 weeks