Re: [PATCH v2] loop: Limit the number of requests in the bio list

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 16 Oct 2012 11:21:45 +0200
Lukas Czerner <lczerner@redhat.com> wrote:

> Currently there is not limitation of number of requests in the loop bio
> list. This can lead into some nasty situations when the caller spawns
> tons of bio requests taking huge amount of memory. This is even more
> obvious with discard where blkdev_issue_discard() will submit all bios
> for the range and wait for them to finish afterwards. On really big loop
> devices and slow backing file system this can lead to OOM situation as
> reported by Dave Chinner.
> 
> With this patch we will wait in loop_make_request() if the number of
> bios in the loop bio list would exceed 'nr_requests' number of requests.
> We'll wake up the process as we process the bios form the list. Some
> threshold hysteresis is in place to avoid high frequency oscillation.
> 

What's happening with this?

> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -463,6 +463,7 @@ out:
>   */
>  static void loop_add_bio(struct loop_device *lo, struct bio *bio)
>  {
> +	lo->lo_bio_count++;
>  	bio_list_add(&lo->lo_bio_list, bio);
>  }
>  
> @@ -471,6 +472,7 @@ static void loop_add_bio(struct loop_device *lo, struct bio *bio)
>   */
>  static struct bio *loop_get_bio(struct loop_device *lo)
>  {
> +	lo->lo_bio_count--;
>  	return bio_list_pop(&lo->lo_bio_list);
>  }
>  
> @@ -489,6 +491,14 @@ static void loop_make_request(struct request_queue *q, struct bio *old_bio)
>  		goto out;
>  	if (unlikely(rw == WRITE && (lo->lo_flags & LO_FLAGS_READ_ONLY)))
>  		goto out;
> +	if (lo->lo_bio_count >= lo->lo_queue->nr_requests) {
> +		unsigned int nr;
> +		spin_unlock_irq(&lo->lo_lock);
> +		nr = lo->lo_queue->nr_requests - (lo->lo_queue->nr_requests/8);
> +		wait_event_interruptible(lo->lo_req_wait,
> +					 lo->lo_bio_count < nr);
> +		spin_lock_irq(&lo->lo_lock);
> +	}

Two things.

a) wait_event_interruptible() will return immediately if a signal is
   pending (eg, someone hit ^C).  This is not the behaviour you want. 
   If the calling process is always a kernel thread then
   wait_event_interruptible() is OK and is the correct thing to use. 
   Otherwise, it will need to be an uninterruptible sleep.

b) Why is it safe to drop lo_lock here?  What data is that lock protecting?