[Devel] [PATCH rh7 v2] ve/devpts: Force devpts mounting to use @newinstance inside VE

[Cyrill Gorcunov]

Modern systemd based containers (such as fedora-21, centos-7) already
mounting initial devpts filesystem with @newinstance option but it
turned out that ubuntu-14 lts doesn't, which makes restore procedure
to fail because we're using get_exec_env as a namespace mark and the
kernel mounts new superblock for container internally. This is done
to isolate devpts between containers but somehow incomplete: @devpts_sb
should server as a root superblock for VE mount namespace. Fix it
adjusting first mount if no @devpts_sb has been assigned.

https://jira.sw.ru/browse/PSBM-34931

Signed-off-by: Cyrill Gorcunov <gorcunov at virtuozzo.com>
CC: Andrey Vagin <avagin at virtuozzo.com>
CC: Vladimir Davydov <vdavydov at virtuozzo.com>
CC: Konstantin Khorenko <khorenko at virtuozzo.com>
CC: Pavel Emelyanov <xemul at virtuozzo.com>
---

Andrew, would this be more suitable?

 fs/devpts/inode.c |   25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

Index: linux-pcs7.git/fs/devpts/inode.c
===================================================================
--- linux-pcs7.git.orig/fs/devpts/inode.c
+++ linux-pcs7.git/fs/devpts/inode.c
@@ -449,10 +449,29 @@ static struct dentry *devpts_mount(struc
 	    (current_user_ns() != &init_user_ns) && !opts.newinstance)
 		return ERR_PTR(-EINVAL);
 
-	if (opts.newinstance)
+	/*
+	 * When we mount devpts it is somehow tricky because of ours
+	 * virtualization for containers sake.
+	 *
+	 * 1) If container mounts devpts without @newinstance option
+	 *    the superblock should point to virtualized one.
+	 *
+	 * 2) If container mounts devpts with @newinstance option
+	 *    the superblock should point into a newly created one.
+	 *
+	 * Thus we use @devpts_sb as a global root superblock for
+	 * each container (including VE0) so if there is no
+	 * @devpts_sb exist we drop @newinstance option and
+	 * mount it in context of container's mount namespace,
+	 * otherwise the old logic remains -- next calls to
+	 * mount with @newinstance allocate new superblocks.
+	 */
+	if (opts.newinstance && get_exec_env()->devpts_sb) {
 		root = mount_nodev(fs_type, flags, data, devpts_fill_super);
-	else
+	} else {
+		opts.newinstance = 0;
 		root = mount_ns(fs_type, flags, data, get_exec_env(), devpts_fill_super);
+	}
 
 	if (IS_ERR(root))
 		return ERR_CAST(root);
@@ -464,7 +483,7 @@ static struct dentry *devpts_mount(struc
 	if (error)
 		goto out_undo_sget;
 
-	if (!opts.newinstance) {
+	if (!opts.newinstance && !get_exec_env()->devpts_sb) {
 		atomic_inc(&s->s_active);
 		get_exec_env()->devpts_sb = s;
 	}