linux-kernel - Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback

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Message-ID: <20090930121029.GA14918@kernel.dk>
Date:	Wed, 30 Sep 2009 14:10:30 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Mason <chris.mason@...cle.com>,
	Artem Bityutskiy <dedekind1@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

On Wed, Sep 30 2009, Jan Kara wrote:
> > +void bdi_writeback_wait(struct backing_dev_info *bdi, long nr_pages)
> > +{
> > +	struct dirty_throttle_task tt = {
> > +		.nr_pages = nr_pages,
> > +		.complete = COMPLETION_INITIALIZER_ONSTACK(tt.complete),
> > +	};
> > +	struct wb_writeback_args args = {
> > +		.sync_mode	= WB_SYNC_NONE,
> > +		.nr_pages	= LONG_MAX,
> > +		.range_cyclic	= 1,
> > +		.for_background	= 1,
> > +	};
> > +	struct bdi_work work;
> > +
> > +	bdi_work_init(&work, &args);
> > +	work.state |= WS_ONSTACK;
> > +
> > +	/*
> > +	 * make sure we will be waken up by someone
> > +	 */
> > +	bdi_queue_work(bdi, &work);
>   This is wrong, you shouldn't submit the work like this because you'll
> have to wait for completion (wb_clear_pending below is just bogus). You
> should rather do bdi_start_writeback(bdi, NULL, 0).

Indeed, the above will die a horrible death fairly soon. But we can add
some "barrier" like synchronization, if you just wish to wait for
previously submitted work to have been completed.


-- 
Jens Axboe

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