Re: [RFC PATCH 1/3] fs: Add s_instance_id field to superblock for unique identification

[Sargun Dhillon <sargun@sargun.me>]

On Sun, Nov 15, 2020 at 8:58 PM Sargun Dhillon <sargun@sargun.me> wrote:
>
> This assigns a per-boot unique number to each superblock. This allows
> other components to know whether a filesystem has been remounted
> since they last interacted with it.
>
> At every boot it is reset to 0. There is no specific reason it is set to 0,
> other than repeatability versus using some random starting number. Because
> of this, you must store it along some other piece of data which is
> initialized at boot time.
>
> This doesn't have any of the overhead of idr, and a u64 wont wrap any time
> soon. There is no forward lookup requirement, so an idr is not needed.
>
> In the future, we may want to expose this to userspace. Userspace programs
> can benefit from this if they have large chunks of dirty or mmaped memory
> that they're interacting with, and they want to see if that volume has been
> unmounted, and remounted. Along with this, and a mechanism to inspect the
> superblock's errseq a user can determine whether they need to throw away
> their cache or similar. This is another benefit in comparison to just
> using a pointer to the superblock to uniquely identify it.
>
> Although this doesn't expose an ioctl or similar yet, in the future we
> could add an ioctl that allows for fetching the s_instance_id for a given
> cache, and inspection of the errseq associated with that.
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-unionfs@vger.kernel.org
> ---
>  fs/super.c              | 3 +++
>  include/linux/fs.h      | 7 +++++++
>  include/uapi/linux/fs.h | 2 ++
>  3 files changed, 12 insertions(+)
>
> diff --git a/fs/super.c b/fs/super.c
> index 904459b35119..e47ace7f8c3d 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -42,6 +42,7 @@
>
>  static int thaw_super_locked(struct super_block *sb);
>
> +static u64 s_instance_id_counter;
>  static LIST_HEAD(super_blocks);
>  static DEFINE_SPINLOCK(sb_lock);
>
> @@ -546,6 +547,7 @@ struct super_block *sget_fc(struct fs_context *fc,
>         s->s_iflags |= fc->s_iflags;
>         strlcpy(s->s_id, s->s_type->name, sizeof(s->s_id));
>         list_add_tail(&s->s_list, &super_blocks);
> +       s->s_instance_id = s_instance_id_counter++;
>         hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
>         spin_unlock(&sb_lock);
>         get_filesystem(s->s_type);
> @@ -625,6 +627,7 @@ struct super_block *sget(struct file_system_type *type,
>         s->s_type = type;
>         strlcpy(s->s_id, type->name, sizeof(s->s_id));
>         list_add_tail(&s->s_list, &super_blocks);
> +       s->s_instance_id = s_instance_id_counter++;
>         hlist_add_head(&s->s_instances, &type->fs_supers);
>         spin_unlock(&sb_lock);
>         get_filesystem(type);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index dbbeb52ce5f3..642847c3673f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1472,6 +1472,13 @@ struct super_block {
>         char                    s_id[32];       /* Informational name */
>         uuid_t                  s_uuid;         /* UUID */
>
> +       /*
> +        * ID identifying this particular instance of the superblock. It can
> +        * be used to determine if a particular filesystem has been remounted.
> +        * It may be exposed to userspace.
> +        */
> +       u64                     s_instance_id;
> +
>         unsigned int            s_max_links;
>         fmode_t                 s_mode;
>

Hit send a little too quickly. Please ignore this hunk as part of the RFC.
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index f44eb0a04afd..f2b126656c22 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -13,6 +13,7 @@
>  #include <linux/limits.h>
>  #include <linux/ioctl.h>
>  #include <linux/types.h>
> +#include <linux/uuid.h>
>  #ifndef __KERNEL__
>  #include <linux/fscrypt.h>
>  #endif
> @@ -203,6 +204,7 @@ struct fsxattr {
>
>  #define        FS_IOC_GETFLAGS                 _IOR('f', 1, long)
>  #define        FS_IOC_SETFLAGS                 _IOW('f', 2, long)
> +#define FS_IOC_GET_SB_INSTANCE         _IOR('f', 3, uuid_t)
>  #define        FS_IOC_GETVERSION               _IOR('v', 1, long)
>  #define        FS_IOC_SETVERSION               _IOW('v', 2, long)
>  #define FS_IOC_FIEMAP                  _IOWR('f', 11, struct fiemap)
> --
> 2.25.1
>