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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 27 May 2010 07:50:41 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Artem Bityutskiy <dedekind1@...il.com>
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 Tue, May 25, 2010 at 04:49:12PM +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> 
> The 'sync_supers' thread wakes up every 5 seconds (by default) and
> writes back all super blocks. It keeps waking up even if there
> are no dirty super-blocks. For many file-systems the superblock
> becomes dirty very rarely, if ever, so 'sync_supers' does not do
> anything most of the time.
> 
> This patch improves 'sync_supers' and makes sleep if all superblocks
> are clean and there is nothing to do. This helps saving the power.
> This optimization is important for small battery-powered devices.

> +void mark_sb_dirty(struct super_block *sb)
> +{
> +	sb->s_dirty = 1;
> +
> +	spin_lock(&supers_timer_lock);
> +	if (!supers_timer_armed) {
> +		bdi_arm_supers_timer();
> +		supers_timer_armed = 1;
> +	} else if (supers_timer_armed == -1)
> +		supers_timer_armed = 1;
> +	spin_unlock(&supers_timer_lock);
> +}
> +EXPORT_SYMBOL(mark_sb_dirty);

Ouch...   That turns a previously trivial operation into something
much heavier; moreover, I'd rather see serious review of s_dirt
uses.

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/