linux-kernel - Re: [PATCHv4 17/17] writeback: lessen sync

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Message-ID: <1275314820.2678.153.camel@localhost>
Date:	Mon, 31 May 2010 17:07:00 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count

On Thu, 2010-05-27 at 07:50 +0100, Al Viro wrote:
> Note, e.g., that in your series you've touched udf; it can set s_dirt
> until the cows come home, but without ->write_super() it'll be ignored
> by everything in VFS and fs/udf itself never looks at the damn thing.
> 
> A look around it shows fs/sysv, where we never clean the damn flag anymore
> for r/w mounts.  Yes, really (got broken a year ago, nobody noticed).
> 
> Or, e.g., BFS - there we have ->write_super() mark the buffer_head that
> contains on-disk sb dirty, and the only place that sets ->s_dirt is doing
> that immediately after having marked the same bh dirty itself.  Interesting
> place, at that - bfs_fill_super() at r/w mount time...  Note that ->sync_fs()
> there does *not* wait for anything, which is not the right thing to do.
> 
> IOW, this thing is a good topic for code review; I suspect that quite a few
> users might be gone as the result.

Al,

you requested me to review s_dirt usage, well, I'm trying now. One thing
I do not understand is s_dirt serialization, which seems to be just
absent in some FSes. I checked affs and ext2. E.g., affs does:

affs_alloc_block()
{
        mark_buffer_dirty(bh);
        sb->s_dirt = 1;
}

vs

affs_write_super()
{
	affs_commit_super();
	/* YYY: what if sb is marked as dirty right here? */
	sb->s_dirt = 0;
}

vs

/* This wakes up periodically */
sync_super()
{
	if (sb->s_root && sb->s_dirt)
		sb->s_op->write_super(sb);
}

ext2 seems to be doing something similar. It seems to me that FSes
should serialize s_dirt changes somehow, but they don't? Why this is not
a problem?

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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