[PATCHv6 3/5] ext4: remove unnecessary superblock dirtying

[Artem Bityutskiy]

From: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>

This patch changes the '__ext4_handle_dirty_super()' function which submits
the superblock for I/O 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 which happens on every delete operation
   (because we update the 's_last_orphan' superblock field).

If the file-system has journal enabled, the superblock is written via the
journal. We do not modify this path.

If the file-system has no journal, this function, falls back to just marking
the superblock as dirty using the 's_dirt' superblock flag. This means that it
delays the actual superblock I/O submission by 5 seconds (default setting).
Namely, the 'sync_supers()' kernel thread will call 'ext4_write_super()' later
and will actually submit the superblock for I/O.

And this is the behavior this patch modifies: we stop using 's_dirt' and just
mark the superblock buffer as dirty right away. Indeed:

1. It does not add any value to delay the I/O submission for cases 1-3 above.
   They are rare.
2. Case number 4 above depends on whether we have file-system checksumming
   enabled or disables.
   a) If it is disabled (most common scenario), then it is all-right to just
      mark the superblock buffer as dirty right away and it should affect
      performance.
   b) If it is enabled, then we'll end up doing a bit more work on deletion
      because we'll re-calculate superblock checksum every time.

So case 2.b is a bit controversial, but I think it is acceptable. After all, by
enabling checksumming we already sign up for paying the price of calculating
it. The way to improve checksumming performance globally would be to calculate
it just before sending buffers to the I/O queue. We'd need some kind of
call-back which could be registered by file-systems.

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@xxxxxxxxxxxxxxx>
---
 fs/ext4/ext4_jbd2.c |    5 ++---
 fs/ext4/super.c     |    2 +-
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
index 90f7c2e..c19ab6a 100644
--- a/fs/ext4/ext4_jbd2.c
+++ b/fs/ext4/ext4_jbd2.c
@@ -151,11 +151,10 @@ int __ext4_handle_dirty_super(const char *where, unsigned int line,
 		if (err)
 			ext4_journal_abort_handle(where, line, __func__,
 						  bh, handle, err);
-	} else if (now) {
+	} else {
 		ext4_superblock_csum_set(sb,
 				(struct ext4_super_block *)bh->b_data);
 		mark_buffer_dirty(bh);
-	} else
-		sb->s_dirt = 1;
+	}
 	return err;
 }
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index eb7aa3e..a391c53 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -896,7 +896,7 @@ 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) {
-- 
1.7.7.6

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