linux-kernel - Re: [RFC] [PATCH] A clean approach to writeout throttling

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Message-ID: <20071210104749.GC5008@kernel.dk>
Date:	Mon, 10 Dec 2007 11:47:50 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC] [PATCH] A clean approach to writeout throttling

On Wed, Dec 05 2007, Daniel Phillips wrote:
> --- 2.6.24-rc3-mm.clean/block/ll_rw_blk.c	2007-12-04 14:45:25.000000000 -0800
> +++ 2.6.24-rc3-mm/block/ll_rw_blk.c	2007-12-04 14:01:18.000000000 -0800
> @@ -3210,7 +3210,7 @@ static inline int bio_check_eod(struct b
>   */
>  static inline void __generic_make_request(struct bio *bio)
>  {
> -	struct request_queue *q;
> +	request_queue_t *q = bdev_get_queue(bio->bi_bdev);
>  	sector_t old_sector;
>  	int ret, nr_sectors = bio_sectors(bio);
>  	dev_t old_dev;
> @@ -3221,6 +3221,13 @@ static inline void __generic_make_reques
>  	if (bio_check_eod(bio, nr_sectors))
>  		goto end_io;
>  
> +	if (q && q->metric && !bio->bi_queue) {
> +		int need = bio->bi_throttle = q->metric(bio);
> +		bio->bi_queue = q;
> +		/* FIXME: potential race if atomic_sub is called in the middle of condition check */
> +		wait_event_interruptible(q->throttle_wait, atomic_read(&q->available) >= need);
> +		atomic_sub(need, &q->available);
> +	}
>  	/*
>  	 * Resolve the mapping until finished. (drivers are
>  	 * still free to implement/resolve their own stacking
> @@ -3234,7 +3241,6 @@ static inline void __generic_make_reques
>  	do {
>  		char b[BDEVNAME_SIZE];
>  
> -		q = bdev_get_queue(bio->bi_bdev);
>  		if (!q) {
>  			printk(KERN_ERR
>  			       "generic_make_request: Trying to access "

Ehm, this patch is so broken it's not even funny - did you even compile?
You would have noticed the warning on request_queue_t, surely. The big
problem is the last hunk here though, how would that work on stacked
devices? Clue: ->bi_bdev is not const, it can change after a call to
->make_request_fn().

-- 
Jens Axboe

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