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

Lukáš Czerner
2012-11-13T08:49:41Z

On Thu, 8 Nov 2012, Andrew Morton wrote:

> Date: Thu, 8 Nov 2012 11:14:18 -0800
> From: Andrew Morton <akpm@linux-foundation.org>
> To: Lukas Czerner <lczerner@redhat.com>
> Cc: axboe@kernel.dk, dchinner@redhat.com, jmoyer@redhat.com,
> linux-kernel@vger.kernel.org
> 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@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.

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