nilfs2: apply readahead for recovery on mount

This inserts readahead in the recovery code.  The readahead request is
issued per segment while searching the latest super root block.

This will shorten mount time after unclean unmount.  A measurement
shows the recovery time was reduced by more than 60 percent:

 e.g. real  0m11.586s -> 0m3.918s  (x 2.96)
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/recovery.c

@@ -798,6 +798,7 @@ int nilfs_search_super_root(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi,
 	struct nilfs_segsum_info ssi;
 	sector_t pseg_start, pseg_end, sr_pseg_start = 0;
 	sector_t seg_start, seg_end; /* range of full segment (block number) */
+	sector_t b, end;
 	u64 seg_seq;
 	__u64 segnum, nextnum = 0;
 	__u64 cno;
@@ -813,6 +814,11 @@ int nilfs_search_super_root(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi,
 	/* Calculate range of segment */
 	nilfs_get_segment_range(nilfs, segnum, &seg_start, &seg_end);
 
+	/* Read ahead segment */
+	b = seg_start;
+	while (b <= seg_end)
+		sb_breadahead(sbi->s_super, b++);
+
 	for (;;) {
 		/* Load segment summary */
 		ret = load_segment_summary(sbi, pseg_start, seg_seq, &ssi, 1);
@@ -835,14 +841,20 @@ int nilfs_search_super_root(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi,
 		ri->ri_nextnum = nextnum;
 		empty_seg = 0;
 
+		if (!NILFS_SEG_HAS_SR(&ssi) && !scan_newer) {
+			/* This will never happen because a superblock
+			   (last_segment) always points to a pseg
+			   having a super root. */
+			ret = NILFS_SEG_FAIL_CONSISTENCY;
+			goto failed;
+		}
+
+		if (pseg_start == seg_start) {
+			nilfs_get_segment_range(nilfs, nextnum, &b, &end);
+			while (b <= end)
+				sb_breadahead(sbi->s_super, b++);
+		}
 		if (!NILFS_SEG_HAS_SR(&ssi)) {
-			if (!scan_newer) {
-				/* This will never happen because a superblock
-				   (last_segment) always points to a pseg
-				   having a super root. */
-				ret = NILFS_SEG_FAIL_CONSISTENCY;
-				goto failed;
-			}
 			if (!ri->ri_lsegs_start && NILFS_SEG_LOGBGN(&ssi)) {
 				ri->ri_lsegs_start = pseg_start;
 				ri->ri_lsegs_start_seq = seg_seq;