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

Jeff Moyer
2012-11-08T21:33:05Z

Andrew Morton <akpm@linux-foundation.org> writes:

> 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?

Still waiting for review, I guess. I'll have a look.

>> --- 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.

Good catch, this needs fixing.

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

lo_lock is protecting access to state and the bio list. Dropping the
lock looks okay to me.

Cheers,
Jeff