Mounting a container's volumes under its rootfs directory inside the
host mount namespace causes problems with cross-namespace mount
propagation when /var/lib/docker is bind-mounted into the container as a
volume. The mount event propagates into the container's mount namespace,
overmounting the volume, but the propagated unmount events do not fully
reverse the effect. Each archive operation causes the mount table in the
container's mount namespace to grow larger and larger, until the kernel
limiton the number of mounts in a namespace is hit. The only solution to
this issue which is not subject to race conditions or other blocker
caveats is to avoid mounting volumes into the container's rootfs
directory in the host mount namespace in the first place.
Mount the container volumes inside an unshared mount namespace to
prevent any mount events from propagating into any other mount
namespace. Greatly simplify the archiving implementations by also
chrooting into the container rootfs to sidestep the need to resolve
paths in the host.
Signed-off-by: Cory Snider <csnider@mirantis.com>
