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

[Lukáš Czerner]

On Thu, 8 Nov 2012, Andrew Morton wrote:

> Date: Thu, 8 Nov 2012 11:14:18 -0800
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Lukas Czerner <lczerner@xxxxxxxxxx>
> Cc: axboe@xxxxxxxxx, dchinner@xxxxxxxxxx, jmoyer@xxxxxxxxxx,
>     linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2] loop: Limit the number of requests in the bio list
> 
> On Tue, 16 Oct 2012 11:21:45 +0200
> Lukas Czerner <lczerner@xxxxxxxxxx> 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.

Understood, I'll fix that.

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

It is protecting the bio list, lo state, backing file so I think it
is perfectly safe to drop the lock there.

Thanks!
-Lukas
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