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From:   Harshad Shirwadkar <harshadshirwadkar@gmail.com>
To:     linux-ext4@vger.kernel.org
Cc:     tytso@mit.edu, Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Subject: [PATCH v6 05/20] jbd2: disable fast commits if journal is empty
Date:   Wed,  8 Apr 2020 14:55:15 -0700
Message-Id: <20200408215530.25649-5-harshads@google.com>
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From: Harshad Shirwadkar <harshadshirwadkar@gmail.com>

If journal is empty, clear the fast commit flag from the on disk
superblock. With this optimization, chances of running into backward
compatibility issues are reduced.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
---
 fs/jbd2/journal.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 79f015f7bf54..f8f55d0814ea 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1482,6 +1482,7 @@ int jbd2_journal_update_sb_log_tail(journal_t *journal, tid_t tail_tid,
 static void jbd2_mark_journal_empty(journal_t *journal, int write_op)
 {
 	journal_superblock_t *sb = journal->j_superblock;
+	bool had_fast_commit = false;
 
 	BUG_ON(!mutex_is_locked(&journal->j_checkpoint_mutex));
 	lock_buffer(journal->j_sb_buffer);
@@ -1495,9 +1496,20 @@ static void jbd2_mark_journal_empty(journal_t *journal, int write_op)
 
 	sb->s_sequence = cpu_to_be32(journal->j_tail_sequence);
 	sb->s_start    = cpu_to_be32(0);
+	if (jbd2_has_feature_fast_commit(journal)) {
+		/*
+		 * When journal is clean, no need to commit fast commit flag and
+		 * make file system incompatible with older kernels.
+		 */
+		jbd2_clear_feature_fast_commit(journal);
+		had_fast_commit = true;
+	}
 
 	jbd2_write_superblock(journal, write_op);
 
+	if (had_fast_commit)
+		jbd2_set_feature_fast_commit(journal);
+
 	/* Log is no longer empty */
 	write_lock(&journal->j_state_lock);
 	journal->j_flags |= JBD2_FLUSHED;
-- 
2.26.0.110.g2183baf09c-goog