nilfs2: introduce secondary super block

The former versions didn't have extra super blocks.  This improves the
weak point by introducing another super block at unused region in tail of
the partition.

This doesn't break disk format compatibility; older versions just ingore
the secondary super block, and new versions just recover it if it doesn't
exist.  The partition created by an old mkfs may not have unused region,
but in that case, the secondary super block will not be added.

This doesn't make more redundant copies of the super block; it is a future
work.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 4 deletions

diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index d750e48257c9..30fe58778d05 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -49,8 +49,10 @@ enum {
  * @ns_sem: semaphore for shared states
  * @ns_writer_mutex: mutex protecting ns_writer attach/detach
  * @ns_writer_refcount: number of referrers on ns_writer
- * @ns_sbh: buffer head of the on-disk super block
- * @ns_sbp: pointer to the super block data
+ * @ns_sbh: buffer heads of on-disk super blocks
+ * @ns_sbp: pointers to super block data
+ * @ns_sbwtime: previous write time of super blocks
+ * @ns_sbsize: size of valid data in super block
  * @ns_supers: list of nilfs super block structs
  * @ns_seg_seq: segment sequence counter
  * @ns_segnum: index number of the latest full segment.
@@ -101,8 +103,10 @@ struct the_nilfs {
          * - protecting s_dirt in the super_block struct
          *   (see nilfs_write_super) and the following fields.
          */
-        struct buffer_head     *ns_sbh;
-        struct nilfs_super_block *ns_sbp;
+        struct buffer_head     *ns_sbh[2];
+        struct nilfs_super_block *ns_sbp[2];
+        time_t                  ns_sbwtime[2];
+        unsigned                ns_sbsize;
         unsigned                ns_mount_state;
         struct list_head        ns_supers;
 
@@ -182,6 +186,10 @@ THE_NILFS_FNS(INIT, init)
 THE_NILFS_FNS(LOADED, loaded)
 THE_NILFS_FNS(DISCONTINUED, discontinued)
 
+/* Minimum interval of periodical update of superblocks (in seconds) */
+#define NILFS_SB_FREQ           10
+#define NILFS_ALTSB_FREQ        60  /* spare superblock */
+
 void nilfs_set_last_segment(struct the_nilfs *, sector_t, u64, __u64);
 struct the_nilfs *alloc_nilfs(struct block_device *);
 void put_nilfs(struct the_nilfs *);
@@ -190,6 +198,8 @@ int load_nilfs(struct the_nilfs *, struct nilfs_sb_info *);
 int nilfs_count_free_blocks(struct the_nilfs *, sector_t *);
 int nilfs_checkpoint_is_mounted(struct the_nilfs *, __u64, int);
 int nilfs_near_disk_full(struct the_nilfs *);
+void nilfs_fall_back_super_block(struct the_nilfs *);
+void nilfs_swap_super_block(struct the_nilfs *);
 
 
 static inline void get_nilfs(struct the_nilfs *nilfs)