[PATCH v2 2/3] ext4: avoid journaling sb update on error if journal is destroying

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

Presently we always BUG_ON if trying to start a transaction on a journal marked
with JBD2_UNMOUNT, since this should never happen. However, while ltp running
stress tests, it was observed that in case of some error handling paths, it is
possible for update_super_work to start a transaction after the journal is
destroyed eg:

(umount)
ext4_kill_sb
  kill_block_super
    generic_shutdown_super
      sync_filesystem /* commits all txns */
      evict_inodes
        /* might start a new txn */
      ext4_put_super
	flush_work(&sbi->s_sb_upd_work) /* flush the workqueue */
        jbd2_journal_destroy
          journal_kill_thread
            journal->j_flags |= JBD2_UNMOUNT;
          jbd2_journal_commit_transaction
            jbd2_journal_get_descriptor_buffer
              jbd2_journal_bmap
                ext4_journal_bmap
                  ext4_map_blocks
                    ...
                    ext4_inode_error
                      ext4_handle_error
                        schedule_work(&sbi->s_sb_upd_work)

                                               /* work queue kicks in */
                                               update_super_work
                                                 jbd2_journal_start
                                                   start_this_handle
                                                     BUG_ON(journal->j_flags &
                                                            JBD2_UNMOUNT)

Hence, introduce a new sbi flag s_journal_destroying to indicate journal is
destroying only do a journaled (and deferred) update of sb if this flag is not
set. Otherwise, just fallback to an un-journaled commit.

We set sbi->s_journal_destroying = true only after all the FS updates are done
during ext4_put_super() (except a running transaction that will get commited
during jbd2_journal_destroy()). After this point, it is safe to commit the sb
outside the journal as it won't race with a journaled update (refer
2d01ddc86606).

Also, we don't need a similar check in ext4_grp_locked_error since it is only
called from mballoc and AFAICT it would be always valid to schedule work here.

Fixes: 2d01ddc86606 ("ext4: save error info to sb through journal if available")
Reported-by: Mahesh Kumar <maheshkumar657g@gmail.com>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
---
 fs/ext4/ext4.h      | 2 ++
 fs/ext4/ext4_jbd2.h | 8 ++++++++
 fs/ext4/super.c     | 4 +++-
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 2b7d781bfcad..d48e93bd5690 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1728,6 +1728,8 @@ struct ext4_sb_info {
 	 */
 	struct work_struct s_sb_upd_work;
 
+	bool s_journal_destorying;
+
 	/* Atomic write unit values in bytes */
 	unsigned int s_awu_min;
 	unsigned int s_awu_max;
diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
index 9b3c9df02a39..6bd3ca84410d 100644
--- a/fs/ext4/ext4_jbd2.h
+++ b/fs/ext4/ext4_jbd2.h
@@ -437,6 +437,14 @@ static inline int ext4_journal_destroy(struct ext4_sb_info *sbi, journal_t *jour
 {
 	int err = 0;
 
+	/*
+	 * At this point all pending FS updates should be done except a possible
+	 * running transaction (which will commit in jbd2_journal_destroy). It
+	 * is now safe for any new errors to directly commit superblock rather
+	 * than going via journal.
+	 */
+	sbi->s_journal_destorying = true;
+
 	err = jbd2_journal_destroy(journal);
 	sbi->s_journal = NULL;
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 8ad664d47806..31552cf0519a 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -706,7 +706,7 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
 		 * constraints, it may not be safe to do it right here so we
 		 * defer superblock flushing to a workqueue.
 		 */
-		if (continue_fs && journal)
+		if (continue_fs && journal && !EXT4_SB(sb)->s_journal_destorying)
 			schedule_work(&EXT4_SB(sb)->s_sb_upd_work);
 		else
 			ext4_commit_super(sb);
@@ -5311,6 +5311,8 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 	spin_lock_init(&sbi->s_error_lock);
 	INIT_WORK(&sbi->s_sb_upd_work, update_super_work);
 
+	sbi->s_journal_destorying = false;
+
 	err = ext4_group_desc_init(sb, es, logical_sb_block, &first_not_zeroed);
 	if (err)
 		goto failed_mount3;
-- 
2.48.1