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

Andrew Morton
2012-11-08T19:14:21Z

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?