From: Sukadev Bhattiprolu <[EMAIL PROTECTED]>
Subject: [RFC][PATCH 8/8]: Enable multiple mounts of devpts
To support containers, allow multiple instances of devpts filesystem.
But to preserve backward compatibility, provide this support for
multiple-mounts under the new mount option, '-o newmnt'.
IOW, devpts must support both single-mount and multiple-mount semantics.
If the filesystem is mounted without the 'newmnt' option (as in current
start-up scripts) the new mount simply binds to the initial kernel mount
of devpts and thus current behavior is preserved.
If the 'newmnt' option is specified (by new container-startup scripts) a
new instance of the devpts fs is created and any ptys created in this
instance are independent of the ptys in other mounts of devpts.
(Hmm would 'private-mount' be a better name as in MAP_PRIVATE) ?
Eg: A container startup script could do the following:
$ ns_exec -cm /bin/bash
$ umount /dev/pts
$ mount -t devpts -o newmnt lxcpts /dev/pts
$ sshd -p 6710
where 'ns_exec -cm /bin/bash' is calls clone() with CLONE_NEWNS flag
and execs /bin/bash in the child process. A pty created by the sshd
is not visible in the original mount of /dev/pts.
USER-SPACE-IMPACT:
The -onewmnt option is meant to minimize userspace impact. Following
are known impacts.
1. /dev/ptmx symlink to pts/ptmx. This is optional if only single-
mount semantics is desired but is required if multi-mount semantics.
2. /dev/pts fs has a new entry (ptmx device node) that is created/
destroyed automatically.
TODO: Others impacts ?
Implementation note:
See comments in new get_sb_ref() function in fs/super.c
(yes fs/super.c !) on why get_sb_single() cannot be
directly used.
Changelog[v2]:
Support both single-mount and multiple-mount semantics and
provide '-onewmnt' option to select the semantics.
---
fs/devpts/inode.c | 43 +++++++++++++++++++++++++++++++++++++++++--
fs/super.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
include/linux/fs.h | 2 ++
3 files changed, 87 insertions(+), 2 deletions(-)
Index: linux-2.6.26-rc8-mm1/fs/devpts/inode.c
===================================================================
--- linux-2.6.26-rc8-mm1.orig/fs/devpts/inode.c 2008-08-20 17:44:29.000000000
-0700
+++ linux-2.6.26-rc8-mm1/fs/devpts/inode.c 2008-08-20 17:50:42.000000000
-0700
@@ -41,10 +41,11 @@ struct pts_mount_opts {
gid_t gid;
umode_t mode;
umode_t ptmx_mode;
+ int newmnt;
};
enum {
- Opt_uid, Opt_gid, Opt_mode, Opt_ptmx_mode,
+ Opt_uid, Opt_gid, Opt_mode, Opt_ptmx_mode, Opt_newmnt,
Opt_err
};
@@ -53,6 +54,7 @@ static match_table_t tokens = {
{Opt_gid, "gid=%u"},
{Opt_mode, "mode=%o"},
{Opt_ptmx_mode, "ptmx_mode=%o"},
+ { Opt_newmnt, "newmnt" },
{Opt_err, NULL}
};
@@ -84,6 +86,7 @@ static int parse_mount_options(char *dat
opts->gid = 0;
opts->mode = DEVPTS_DEFAULT_MODE;
opts->ptmx_mode = DEVPTS_DEFAULT_PTMX_MODE;
+ opts->newmnt = 0;
while ((p = strsep(&data, ",")) != NULL) {
substring_t args[MAX_OPT_ARGS];
@@ -117,6 +120,9 @@ static int parse_mount_options(char *dat
return -EINVAL;
opts->ptmx_mode = option & S_IALLUGO;
break;
+ case Opt_newmnt:
+ opts->newmnt = 1;
+ break;
default:
printk(KERN_ERR "devpts: called with bogus options\n");
return -EINVAL;
@@ -145,6 +151,8 @@ static int devpts_show_options(struct se
seq_printf(seq, ",gid=%u", opts->gid);
seq_printf(seq, ",mode=%03o", opts->mode);
seq_printf(seq, ",ptmx_mode=%03o", opts->ptmx_mode);
+ if (opts->newmnt)
+ seq_printf(seq, ",newmnt");
return 0;
}
@@ -256,12 +264,43 @@ int mknod_ptmx(struct super_block *sb)
return 0;
}
+static int mount_init_pts(struct file_system_type *fs_type, int flags,
+ void *data, struct vfsmount *mnt)
+{
+ int err;
+
+ if (!devpts_mnt) {
+ err = get_sb_single(fs_type, flags, data, devpts_fill_super,
+ mnt);
+ if (!err)
+ devpts_mnt = mnt;
+ return err;
+ }
+
+ err = get_sb_ref(devpts_mnt->mnt_sb, flags, data, mnt);
+
+ printk(KERN_ERR "mount_init_pts(): returning %d\n", err);
+ return err;
+}
+
static int devpts_get_sb(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data, struct vfsmount *mnt)
{
int err;
+ struct pts_mount_opts opts;
+
+ if (parse_mount_options((char *)data, &opts))
+ return -EINVAL;
+
+ printk(KERN_ERR "devpts_get_sb(): newmnt option is %d\n", opts.newmnt);
+
+ if (opts.newmnt) {
+ err = get_sb_nodev(fs_type, flags, data, devpts_fill_super,
+ mnt);
+ } else {
+ err = mount_init_pts(fs_type, flags, data, mnt);
+ }
- err = get_sb_single(fs_type, flags, data, devpts_fill_super, mnt);
if (err)
return err;
Index: linux-2.6.26-rc8-mm1/fs/super.c
===================================================================
--- linux-2.6.26-rc8-mm1.orig/fs/super.c 2008-08-20 17:44:29.000000000
-0700
+++ linux-2.6.26-rc8-mm1/fs/super.c 2008-08-20 18:07:38.000000000 -0700
@@ -883,6 +883,50 @@ int get_sb_single(struct file_system_typ
EXPORT_SYMBOL(get_sb_single);
+int get_sb_ref(struct super_block *sb, int flags, void *data,
+ struct vfsmount *mnt)
+{
+ int err;
+
+ /*
+ * UGLY:
+ *
+ * This is needed to support multiple mounts in devpts while
+ * preserving backward compatibility of the current 'single-mount'
+ * semantics.
+ *
+ * devpts cannot simply use get_sb_single(), bc get_sb_single() or
+ * more specifically, sget() finds the most recent mount of devpts.
+ * But that recent mount may not the be initial kernel mount (user
+ * may mounted with the '-onewmnt' option since the initial mount
+ * and get_sb_single() would pick that super-block).
+ *
+ * Caller is responsible to ensure that 'sb' is valid initialized.
+ * So armed with that fact, unroll essentials of get_sb_single()
+ * here.
+ */
+ spin_lock(&sb_lock);
+
+ if (!grab_super(sb)) {
+ /*
+ * TODO: anymore cleanup ?
+ */
+ return -EAGAIN;
+ }
+
+ err = do_remount_sb(sb, flags, data, 0);
+ if (err) {
+ /*
+ * (don't deactivate_super() here - its from initial pts mount)
+ *
+ * TODO: anymore cleanup ?
+ */
+ up_write(&sb->s_umount);
+ return err;
+ }
+ return simple_set_mnt(mnt, sb);
+}
+
struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name,
void *data)
{
Index: linux-2.6.26-rc8-mm1/include/linux/fs.h
===================================================================
--- linux-2.6.26-rc8-mm1.orig/include/linux/fs.h 2008-08-20
17:46:27.000000000 -0700
+++ linux-2.6.26-rc8-mm1/include/linux/fs.h 2008-08-20 17:47:04.000000000
-0700
@@ -1522,6 +1522,8 @@ extern int get_sb_nodev(struct file_syst
int flags, void *data,
int (*fill_super)(struct super_block *, void *, int),
struct vfsmount *mnt);
+extern int get_sb_ref(struct super_block *sb, int flags, void *data,
+ struct vfsmount *mnt);
void generic_shutdown_super(struct super_block *sb);
void kill_block_super(struct super_block *sb);
void kill_anon_super(struct super_block *sb);
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