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    Subject[PATCH 04/19] vfs: Add mount change counter [ver #16]
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    Add a change counter on each mount object so that the user can easily check
    to see if a mount has changed its attributes or its children.

    Future patches will:

    (1) Provide this value through fsinfo() attributes.

    (2) Implement a watch_mount() system call to provide a notification
    interface for userspace monitoring.

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    fs/mount.h | 22 ++++++++++++++++++++++
    fs/namespace.c | 11 +++++++++++
    2 files changed, 33 insertions(+)

    diff --git a/fs/mount.h b/fs/mount.h
    index 711a4093e475..a1625924fe81 100644
    --- a/fs/mount.h
    +++ b/fs/mount.h
    @@ -72,6 +72,7 @@ struct mount {
    int mnt_expiry_mark; /* true if marked for expiry */
    struct hlist_head mnt_pins;
    struct hlist_head mnt_stuck_children;
    + atomic_t mnt_change_counter; /* Number of changed applied */
    } __randomize_layout;

    #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
    @@ -153,3 +154,24 @@ static inline bool is_anon_ns(struct mnt_namespace *ns)
    {
    return ns->seq == 0;
    }
    +
    +/*
    + * Type of mount topology change notification.
    + */
    +enum mount_notification_subtype {
    + NOTIFY_MOUNT_NEW_MOUNT = 0, /* New mount added */
    + NOTIFY_MOUNT_UNMOUNT = 1, /* Mount removed manually */
    + NOTIFY_MOUNT_EXPIRY = 2, /* Automount expired */
    + NOTIFY_MOUNT_READONLY = 3, /* Mount R/O state changed */
    + NOTIFY_MOUNT_SETATTR = 4, /* Mount attributes changed */
    + NOTIFY_MOUNT_MOVE_FROM = 5, /* Mount moved from here */
    + NOTIFY_MOUNT_MOVE_TO = 6, /* Mount moved to here (compare op_id) */
    +};
    +
    +static inline void notify_mount(struct mount *changed,
    + struct mount *aux,
    + enum mount_notification_subtype subtype,
    + u32 info_flags)
    +{
    + atomic_inc(&changed->mnt_change_counter);
    +}
    diff --git a/fs/namespace.c b/fs/namespace.c
    index 85b5f7bea82e..5c84aadb6aa1 100644
    --- a/fs/namespace.c
    +++ b/fs/namespace.c
    @@ -498,6 +498,8 @@ static int mnt_make_readonly(struct mount *mnt)
    smp_wmb();
    mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
    unlock_mount_hash();
    + if (ret == 0)
    + notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY, 0x10000);
    return ret;
    }

    @@ -506,6 +508,7 @@ static int __mnt_unmake_readonly(struct mount *mnt)
    lock_mount_hash();
    mnt->mnt.mnt_flags &= ~MNT_READONLY;
    unlock_mount_hash();
    + notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY, 0);
    return 0;
    }

    @@ -819,6 +822,7 @@ static struct mountpoint *unhash_mnt(struct mount *mnt)
    */
    static void umount_mnt(struct mount *mnt)
    {
    + notify_mount(mnt->mnt_parent, mnt, NOTIFY_MOUNT_UNMOUNT, 0);
    put_mountpoint(unhash_mnt(mnt));
    }

    @@ -1453,6 +1457,7 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
    p = list_first_entry(&tmp_list, struct mount, mnt_list);
    list_del_init(&p->mnt_expire);
    list_del_init(&p->mnt_list);
    +
    ns = p->mnt_ns;
    if (ns) {
    ns->mounts--;
    @@ -2078,7 +2083,10 @@ static int attach_recursive_mnt(struct mount *source_mnt,
    lock_mount_hash();
    }
    if (moving) {
    + notify_mount(source_mnt->mnt_parent, source_mnt,
    + NOTIFY_MOUNT_MOVE_FROM, 0);
    unhash_mnt(source_mnt);
    + notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_MOVE_TO, 0);
    attach_mnt(source_mnt, dest_mnt, dest_mp);
    touch_mnt_namespace(source_mnt->mnt_ns);
    } else {
    @@ -2087,6 +2095,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
    list_del_init(&source_mnt->mnt_ns->list);
    }
    mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
    + notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_NEW_MOUNT, 0);
    commit_tree(source_mnt);
    }

    @@ -2464,6 +2473,7 @@ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
    mnt->mnt.mnt_flags = mnt_flags;
    touch_mnt_namespace(mnt->mnt_ns);
    unlock_mount_hash();
    + notify_mount(mnt, NULL, NOTIFY_MOUNT_SETATTR, 0);
    }

    static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
    @@ -2898,6 +2908,7 @@ void mark_mounts_for_expiry(struct list_head *mounts)
    if (!xchg(&mnt->mnt_expiry_mark, 1) ||
    propagate_mount_busy(mnt, 1))
    continue;
    + notify_mount(mnt, NULL, NOTIFY_MOUNT_EXPIRY, 0);
    list_move(&mnt->mnt_expire, &graveyard);
    }
    while (!list_empty(&graveyard)) {

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