[PATCHv4 3/5] ext4: remove unnecessary superblock dirtying

[Artem Bityutskiy <dedekind1@gmail.com>]

From: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

This patch changes the '__ext4_handle_dirty_super()' function which is used
by ext4 to update the superblock via the journal in the following cases:

1. When creating the first large file on a file system without
   EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature.
2. When re-sizing the file-system.
3. When creating an xattr on a file-system without the
   EXT4_FEATURE_COMPAT_EXT_ATTR feature.
4. When adding or deleting an orphan (because we update the 's_last_orphan'
   superblock field).

This function, however, falls back to just marking the superblock as dirty
if the file-system has no journal. This means that we delay the actual
superblock I/O submission by 5 seconds (roughly speaking). Namely, the
'sync_supers()' kernel thread will call 'ext4_write_super()' later, where
we actually will submit the superblock down to the media.

However:
1. For cases 1-3 it does not add any value to delay the I/O submission. These
   events are rare and we may just commit submit the superblock for
   asynchronous I/O right away.
2. For case 4 - similarly, not terribly frequent event in most of workloads.
   It should be good enough to just submit asynchronous superblock write-out.

This patch also removes 's_dirt' condition on the unmount path because we never
set it anymore, so we should not test it.

Tested using xfstests for both journalled and non-journalled ext4.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
---
 fs/ext4/ext4.h      |    1 +
 fs/ext4/ext4_jbd2.c |    2 +-
 fs/ext4/super.c     |    5 ++---
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 0c4042e..b2439d5 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2041,6 +2041,7 @@ extern int ext4_superblock_csum_verify(struct super_block *sb,
 				       struct ext4_super_block *es);
 extern void ext4_superblock_csum_set(struct super_block *sb,
 				     struct ext4_super_block *es);
+extern int ext4_commit_super(struct super_block *sb, int sync);
 extern void *ext4_kvmalloc(size_t size, gfp_t flags);
 extern void *ext4_kvzalloc(size_t size, gfp_t flags);
 extern void ext4_kvfree(void *ptr);
diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
index 90f7c2e..27354df 100644
--- a/fs/ext4/ext4_jbd2.c
+++ b/fs/ext4/ext4_jbd2.c
@@ -156,6 +156,6 @@ int __ext4_handle_dirty_super(const char *where, unsigned int line,
 				(struct ext4_super_block *)bh->b_data);
 		mark_buffer_dirty(bh);
 	} else
-		sb->s_dirt = 1;
+		err = ext4_commit_super(sb, 0);
 	return err;
 }
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index eb7aa3e..9b26ba0 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -63,7 +63,6 @@ static struct ext4_features *ext4_feat;
 static int ext4_load_journal(struct super_block *, struct ext4_super_block *,
 			     unsigned long journal_devnum);
 static int ext4_show_options(struct seq_file *seq, struct dentry *root);
-static int ext4_commit_super(struct super_block *sb, int sync);
 static void ext4_mark_recovery_complete(struct super_block *sb,
 					struct ext4_super_block *es);
 static void ext4_clear_journal_err(struct super_block *sb,
@@ -896,7 +895,7 @@ static void ext4_put_super(struct super_block *sb)
 		EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER);
 		es->s_state = cpu_to_le16(sbi->s_mount_state);
 	}
-	if (sb->s_dirt || !(sb->s_flags & MS_RDONLY))
+	if (!(sb->s_flags & MS_RDONLY))
 		ext4_commit_super(sb, 1);
 
 	if (sbi->s_proc) {
@@ -4155,7 +4154,7 @@ static int ext4_load_journal(struct super_block *sb,
 	return 0;
 }
 
-static int ext4_commit_super(struct super_block *sb, int sync)
+int ext4_commit_super(struct super_block *sb, int sync)
 {
 	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
 	struct buffer_head *sbh = EXT4_SB(sb)->s_sbh;
-- 
1.7.7.6