linux-kernel - Re: [PATCH] Optimize wait_sb

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Date:	Wed, 26 Jun 2013 10:32:22 -0400
From:	Jörn Engel <joern@...fs.org>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tux3@...3.org
Subject: Re: [PATCH] Optimize wait_sb_inodes()

On Wed, 26 June 2013 17:45:23 +0900, OGAWA Hirofumi wrote:
>
> 
>  fs/fs-writeback.c  |    5 ++++-
>  include/linux/fs.h |    1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff -puN include/linux/fs.h~tux3-fix-wait_sb_inodes include/linux/fs.h
> --- tux3fs/include/linux/fs.h~tux3-fix-wait_sb_inodes	2013-06-24 19:03:10.000000000 +0900
> +++ tux3fs-hirofumi/include/linux/fs.h	2013-06-24 19:03:10.000000000 +0900
> @@ -1595,6 +1595,7 @@ struct super_operations {
>  	int (*write_inode) (struct inode *, struct writeback_control *wbc);
>  	int (*drop_inode) (struct inode *);
>  	void (*evict_inode) (struct inode *);
> +	void (*wait_inodes)(struct super_block *);
>  	void (*put_super) (struct super_block *);
>  	int (*sync_fs)(struct super_block *sb, int wait);
>  	int (*freeze_fs) (struct super_block *);
> diff -puN fs/fs-writeback.c~tux3-fix-wait_sb_inodes fs/fs-writeback.c
> --- tux3fs/fs/fs-writeback.c~tux3-fix-wait_sb_inodes	2013-06-24 19:03:10.000000000 +0900
> +++ tux3fs-hirofumi/fs/fs-writeback.c	2013-06-24 19:03:10.000000000 +0900
> @@ -1401,7 +1401,10 @@ void sync_inodes_sb(struct super_block *
>  	bdi_queue_work(sb->s_bdi, &work);
>  	wait_for_completion(&done);
>  
> -	wait_sb_inodes(sb);
> +	if (sb->s_op->wait_inodes)
> +		sb->s_op->wait_inodes(sb);
> +	else
> +		wait_sb_inodes(sb);
>  }
>  EXPORT_SYMBOL(sync_inodes_sb);

Two things.  Until there are actual implementations of
s_op->wait_inodes, this is pure obfuscation.  You already know this,
of course.

More interestingly, I personally hate methods with a default option if
they are not implemented.  Not too bad in this particular case, but
the same pattern has burned me a number of times and wasted weeks of
my life.  So I would prefer to unconditionally call
sb->s_op->wait_inodes(sb) and set it to wait_sb_inodes for all
filesystems that don't have a smarter way to do things.

Jörn

--
Linux is more the core point of a concept that surrounds "open source"
which, in turn, is based on a false concept. This concept is that
people actually want to look at source code.
-- Rob Enderle
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