ext4: make num of fast commit blocks configurable

This patch reserves a field in the jbd2 superblock for number of fast
commit blocks. When this value is non-zero, Ext4 uses this field to
set the number of fast commit blocks.

Fixes: 6866d7b3 ("ext4/jbd2: add fast commit initialization")
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201027044915.2553163-2-harshadshirwadkar@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/fast_commit.c

@@ -2077,6 +2077,8 @@ static int ext4_fc_replay(journal_t *journal, struct buffer_head *bh,
 
 void ext4_fc_init(struct super_block *sb, journal_t *journal)
 {
+	int num_fc_blocks;
+
 	/*
 	 * We set replay callback even if fast commit disabled because we may
 	 * could still have fast commit blocks that need to be replayed even if
@@ -2086,7 +2088,15 @@ void ext4_fc_init(struct super_block *sb, journal_t *journal)
 	if (!test_opt2(sb, JOURNAL_FAST_COMMIT))
 		return;
 	journal->j_fc_cleanup_callback = ext4_fc_cleanup;
-	if (jbd2_fc_init(journal, EXT4_NUM_FC_BLKS)) {
+	if (!buffer_uptodate(journal->j_sb_buffer)
+		&& ext4_read_bh_lock(journal->j_sb_buffer, REQ_META | REQ_PRIO,
+					true)) {
+		ext4_msg(sb, KERN_ERR, "I/O error on journal");
+		return;
+	}
+	num_fc_blocks = be32_to_cpu(journal->j_superblock->s_num_fc_blks);
+	if (jbd2_fc_init(journal, num_fc_blocks ? num_fc_blocks :
+					EXT4_NUM_FC_BLKS)) {
 		pr_warn("Error while enabling fast commits, turning off.");
 		ext4_clear_feature_fast_commit(sb);
 	}

include/linux/jbd2.h

@@ -263,7 +263,10 @@ typedef struct journal_superblock_s
 /* 0x0050 */
 	__u8	s_checksum_type;	/* checksum type */
 	__u8	s_padding2[3];
-	__u32	s_padding[42];
+/* 0x0054 */
+	__be32	s_num_fc_blks;		/* Number of fast commit blocks */
+/* 0x0058 */
+	__u32	s_padding[41];
 	__be32	s_checksum;		/* crc32c(superblock) */
 
 /* 0x0100 */