ext4: Track lifetime disk writes

Add a new superblock value which tracks the lifetime amount of writes
to the filesystem.  This is useful in estimating the amount of wear on
solid state drives (SSD's) caused by writes to the filesystem.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -683,7 +683,8 @@ struct ext4_super_block {
 	__u8	s_log_groups_per_flex;  /* FLEX_BG group size */
 	__u8	s_reserved_char_pad2;
 	__le16  s_reserved_pad;
-	__u32   s_reserved[162];        /* Padding to the end of the block */
+	__le64	s_kbytes_written;	/* nr of lifetime kilobytes written */
+	__u32   s_reserved[160];        /* Padding to the end of the block */
 };
 
 #ifdef __KERNEL__

fs/ext4/ext4_sb.h

@@ -142,6 +142,10 @@ struct ext4_sb_info {
 	/* locality groups */
 	struct ext4_locality_group *s_locality_groups;
 
+	/* for write statistics */
+	unsigned long s_sectors_written_start;
+	u64 s_kbytes_written;
+
 	unsigned int s_log_groups_per_flex;
 	struct flex_groups *s_flex_groups;
 };

fs/ext4/super.c

@@ -2035,6 +2035,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_resgid = EXT4_DEF_RESGID;
 	sbi->s_inode_readahead_blks = EXT4_DEF_INODE_READAHEAD_BLKS;
 	sbi->s_sb_block = sb_block;
+	sbi->s_sectors_written_start = part_stat_read(sb->s_bdev->bd_part,
+						      sectors[1]);
 
 	unlock_kernel();
 
@@ -2072,6 +2074,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sb->s_magic = le16_to_cpu(es->s_magic);
 	if (sb->s_magic != EXT4_SUPER_MAGIC)
 		goto cantfind_ext4;
+	sbi->s_kbytes_written = le64_to_cpu(es->s_kbytes_written);
 
 	/* Set defaults before we parse the mount options */
 	def_mount_opts = le32_to_cpu(es->s_default_mount_opts);
@@ -2921,6 +2924,10 @@ static int ext4_commit_super(struct super_block *sb,
 		set_buffer_uptodate(sbh);
 	}
 	es->s_wtime = cpu_to_le32(get_seconds());
+	es->s_kbytes_written =
+		cpu_to_le64(EXT4_SB(sb)->s_kbytes_written + 
+			    ((part_stat_read(sb->s_bdev->bd_part, sectors[1]) -
+			      EXT4_SB(sb)->s_sectors_written_start) >> 1));
 	ext4_free_blocks_count_set(es, percpu_counter_sum_positive(
 					&EXT4_SB(sb)->s_freeblocks_counter));
 	es->s_free_inodes_count = cpu_to_le32(percpu_counter_sum_positive(