ext4: clean up the JBD2 API that initializes fast commits

This patch removes jbd2_fc_init() API and its related functions to
simplify enabling fast commits. With this change, the number of fast
commit blocks to use is solely determined by the JBD2 layer. So, we
move the default value for minimum number of fast commit blocks from
ext4/fast_commit.h to include/linux/jbd2.h. However, whether or not to
use fast commits is determined by the file system. The file system
just sets the fast commit feature using
jbd2_journal_set_features(). JBD2 layer then determines how many
blocks to use for fast commits (based on the value found in the JBD2
superblock).

Note that the JBD2 feature flag of fast commits is just an indication
that there are fast commit blocks present on disk. It doesn't tell
JBD2 layer about the intent of the file system of whether to it wants
to use fast commit or not. That's why, we blindly clear the fast
commit flag in journal_reset() after the recovery is done.
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-7-harshadshirwadkar@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/fast_commit.c

@@ -2091,8 +2091,6 @@ 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
@@ -2102,18 +2100,6 @@ 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 (!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);
-	}
 }
 
 const char *fc_ineligible_reasons[] = {

fs/ext4/fast_commit.h

@@ -3,9 +3,6 @@
 #ifndef __FAST_COMMIT_H__
 #define __FAST_COMMIT_H__
 
-/* Number of blocks in journal area to allocate for fast commits */
-#define EXT4_NUM_FC_BLKS		256
-
 /* Fast commit tags */
 #define EXT4_FC_TAG_ADD_RANGE		0x0001
 #define EXT4_FC_TAG_DEL_RANGE		0x0002

fs/ext4/super.c

@@ -4857,6 +4857,14 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		goto failed_mount_wq;
 	}
 
+	if (test_opt2(sb, JOURNAL_FAST_COMMIT) &&
+		!jbd2_journal_set_features(EXT4_SB(sb)->s_journal, 0, 0,
+					  JBD2_FEATURE_INCOMPAT_FAST_COMMIT)) {
+		ext4_msg(sb, KERN_ERR,
+			"Failed to set fast commit journal feature");
+		goto failed_mount_wq;
+	}
+
 	/* We have now updated the journal if required, so we can
 	 * validate the data journaling mode. */
 	switch (test_opt(sb, DATA_FLAGS)) {

fs/jbd2/journal.c

@@ -1352,19 +1352,12 @@ static journal_t *journal_init_common(struct block_device *bdev,
 	/* We need enough buffers to write out full descriptor block. */
 	n = journal->j_blocksize / jbd2_min_tag_size();
 	journal->j_wbufsize = n;
+	journal->j_fc_wbuf = NULL;
 	journal->j_wbuf = kmalloc_array(n, sizeof(struct buffer_head *),
 					GFP_KERNEL);
 	if (!journal->j_wbuf)
 		goto err_cleanup;
 
-	if (journal->j_fc_wbufsize > 0) {
-		journal->j_fc_wbuf = kmalloc_array(journal->j_fc_wbufsize,
-					sizeof(struct buffer_head *),
-					GFP_KERNEL);
-		if (!journal->j_fc_wbuf)
-			goto err_cleanup;
-	}
-
 	bh = getblk_unmovable(journal->j_dev, start, journal->j_blocksize);
 	if (!bh) {
 		pr_err("%s: Cannot get buffer for journal superblock\n",
@@ -1378,23 +1371,11 @@ static journal_t *journal_init_common(struct block_device *bdev,
 
 err_cleanup:
 	kfree(journal->j_wbuf);
-	kfree(journal->j_fc_wbuf);
 	jbd2_journal_destroy_revoke(journal);
 	kfree(journal);
 	return NULL;
 }
 
-int jbd2_fc_init(journal_t *journal, int num_fc_blks)
-{
-	journal->j_fc_wbufsize = num_fc_blks;
-	journal->j_fc_wbuf = kmalloc_array(journal->j_fc_wbufsize,
-				sizeof(struct buffer_head *), GFP_KERNEL);
-	if (!journal->j_fc_wbuf)
-		return -ENOMEM;
-	return 0;
-}
-EXPORT_SYMBOL(jbd2_fc_init);
-
 /* jbd2_journal_init_dev and jbd2_journal_init_inode:
  *
  * Create a journal structure assigned some fixed set of disk blocks to
@@ -1512,16 +1493,7 @@ static int journal_reset(journal_t *journal)
 	}
 
 	journal->j_first = first;
-
-	if (jbd2_has_feature_fast_commit(journal) &&
-	    journal->j_fc_wbufsize > 0) {
-		journal->j_fc_last = last;
-		journal->j_last = last - journal->j_fc_wbufsize;
-		journal->j_fc_first = journal->j_last + 1;
-		journal->j_fc_off = 0;
-	} else {
-		journal->j_last = last;
-	}
+	journal->j_last = last;
 
 	journal->j_head = journal->j_first;
 	journal->j_tail = journal->j_first;
@@ -1533,6 +1505,13 @@ static int journal_reset(journal_t *journal)
 
 	journal->j_max_transaction_buffers = jbd2_journal_get_max_txn_bufs(journal);
 
+	/*
+	 * Now that journal recovery is done, turn fast commits off here. This
+	 * way, if fast commit was enabled before the crash but if now FS has
+	 * disabled it, we don't enable fast commits.
+	 */
+	jbd2_clear_feature_fast_commit(journal);
+
 	/*
 	 * As a special case, if the on-disk copy is already marked as needing
 	 * no recovery (s_start == 0), then we can safely defer the superblock
@@ -1872,6 +1851,7 @@ static int load_superblock(journal_t *journal)
 {
 	int err;
 	journal_superblock_t *sb;
+	int num_fc_blocks;
 
 	err = journal_get_superblock(journal);
 	if (err)
@@ -1883,15 +1863,17 @@ static int load_superblock(journal_t *journal)
 	journal->j_tail = be32_to_cpu(sb->s_start);
 	journal->j_first = be32_to_cpu(sb->s_first);
 	journal->j_errno = be32_to_cpu(sb->s_errno);
+	journal->j_last = be32_to_cpu(sb->s_maxlen);
 
-	if (jbd2_has_feature_fast_commit(journal) &&
-	    journal->j_fc_wbufsize > 0) {
+	if (jbd2_has_feature_fast_commit(journal)) {
 		journal->j_fc_last = be32_to_cpu(sb->s_maxlen);
-		journal->j_last = journal->j_fc_last - journal->j_fc_wbufsize;
+		num_fc_blocks = be32_to_cpu(sb->s_num_fc_blks);
+		if (!num_fc_blocks)
+			num_fc_blocks = JBD2_MIN_FC_BLOCKS;
+		if (journal->j_last - num_fc_blocks >= JBD2_MIN_JOURNAL_BLOCKS)
+			journal->j_last = journal->j_fc_last - num_fc_blocks;
 		journal->j_fc_first = journal->j_last + 1;
 		journal->j_fc_off = 0;
-	} else {
-		journal->j_last = be32_to_cpu(sb->s_maxlen);
 	}
 
 	return 0;
@@ -1954,9 +1936,6 @@ int jbd2_journal_load(journal_t *journal)
 	 */
 	journal->j_flags &= ~JBD2_ABORT;
 
-	if (journal->j_fc_wbufsize > 0)
-		jbd2_journal_set_features(journal, 0, 0,
-					  JBD2_FEATURE_INCOMPAT_FAST_COMMIT);
 	/* OK, we've finished with the dynamic journal bits:
 	 * reinitialise the dynamic contents of the superblock in memory
 	 * and reset them on disk. */
@@ -2040,8 +2019,7 @@ int jbd2_journal_destroy(journal_t *journal)
 		jbd2_journal_destroy_revoke(journal);
 	if (journal->j_chksum_driver)
 		crypto_free_shash(journal->j_chksum_driver);
-	if (journal->j_fc_wbufsize > 0)
-		kfree(journal->j_fc_wbuf);
+	kfree(journal->j_fc_wbuf);
 	kfree(journal->j_wbuf);
 	kfree(journal);
 
@@ -2116,6 +2094,37 @@ int jbd2_journal_check_available_features(journal_t *journal, unsigned long comp
 	return 0;
 }
 
+static int
+jbd2_journal_initialize_fast_commit(journal_t *journal)
+{
+	journal_superblock_t *sb = journal->j_superblock;
+	unsigned long long num_fc_blks;
+
+	num_fc_blks = be32_to_cpu(sb->s_num_fc_blks);
+	if (num_fc_blks == 0)
+		num_fc_blks = JBD2_MIN_FC_BLOCKS;
+	if (journal->j_last - num_fc_blks < JBD2_MIN_JOURNAL_BLOCKS)
+		return -ENOSPC;
+
+	/* Are we called twice? */
+	WARN_ON(journal->j_fc_wbuf != NULL);
+	journal->j_fc_wbuf = kmalloc_array(num_fc_blks,
+				sizeof(struct buffer_head *), GFP_KERNEL);
+	if (!journal->j_fc_wbuf)
+		return -ENOMEM;
+
+	journal->j_fc_wbufsize = num_fc_blks;
+	journal->j_fc_last = journal->j_last;
+	journal->j_last = journal->j_fc_last - num_fc_blks;
+	journal->j_fc_first = journal->j_last + 1;
+	journal->j_fc_off = 0;
+	journal->j_free = journal->j_last - journal->j_first;
+	journal->j_max_transaction_buffers =
+		jbd2_journal_get_max_txn_bufs(journal);
+
+	return 0;
+}
+
 /**
  * int jbd2_journal_set_features() - Mark a given journal feature in the superblock
  * @journal: Journal to act on.
@@ -2159,6 +2168,13 @@ int jbd2_journal_set_features(journal_t *journal, unsigned long compat,
 
 	sb = journal->j_superblock;
 
+	if (incompat & JBD2_FEATURE_INCOMPAT_FAST_COMMIT) {
+		if (jbd2_journal_initialize_fast_commit(journal)) {
+			pr_err("JBD2: Cannot enable fast commits.\n");
+			return 0;
+		}
+	}
+
 	/* Load the checksum driver if necessary */
 	if ((journal->j_chksum_driver == NULL) &&
 	    INCOMPAT_FEATURE_ON(JBD2_FEATURE_INCOMPAT_CSUM_V3)) {

include/linux/jbd2.h

@@ -68,6 +68,7 @@ extern void *jbd2_alloc(size_t size, gfp_t flags);
 extern void jbd2_free(void *ptr, size_t size);
 
 #define JBD2_MIN_JOURNAL_BLOCKS 1024
+#define JBD2_MIN_FC_BLOCKS	256
 
 #ifdef __KERNEL__
 
@@ -1614,7 +1615,6 @@ extern void __jbd2_journal_drop_transaction(journal_t *, transaction_t *);
 extern int jbd2_cleanup_journal_tail(journal_t *);
 
 /* Fast commit related APIs */
-int jbd2_fc_init(journal_t *journal, int num_fc_blks);
 int jbd2_fc_begin_commit(journal_t *journal, tid_t tid);
 int jbd2_fc_end_commit(journal_t *journal);
 int jbd2_fc_end_commit_fallback(journal_t *journal, tid_t tid);