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Date:	Wed, 4 Jul 2012 15:11:24 +0200
From:	Jan Kara <jack@...e.cz>
To:	Artem Bityutskiy <dedekind1@...il.com>
Cc:	Theodore Tso <tytso@....edu>, Jan Kara <jack@...e.cz>,
	Linux FS Maling List <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Maling List <linux-kernel@...r.kernel.org>,
	Ext4 Mailing List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCHv4 3/5] ext4: remove unnecessary superblock dirtying

On Wed 04-07-12 15:21:52, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <artem.bityutskiy@...ux.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.
  Well, it happens for every inode being truncated / deleted to it can be
rather frequent. That's why I wanted to have now == 1 case everywhere -
i.e. just recompute the checksum and do mark_buffer_dirty(). I'd just
remove the 'now' test in this patch and then in patch 5 remove the now
argument from the function and callers as you did.

									Honza

> 
> 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@...ux.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
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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