Re: block: fix blk_queue_split() resource exhaustion

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Jun 24 2016 at 10:27am -0400,
Lars Ellenberg <lars.ellenberg@linbit.com> wrote:

> On Fri, Jun 24, 2016 at 07:36:57PM +0800, Ming Lei wrote:
> > >
> > > This is not a theoretical problem.
> > > At least int DRBD, and an unfortunately high IO concurrency wrt. the
> > > "max-buffers" setting, without this patch we have a reproducible deadlock.
> > 
> > Is there any log about the deadlock? And is there any lockdep warning
> > if it is enabled?
> 
> In DRBD, to avoid potentially very long internal queues as we wait for
> our replication peer device and local backend, we limit the number of
> in-flight bios we accept, and block in our ->make_request_fn() if that
> number exceeds a configured watermark ("max-buffers").
> 
> Works fine, as long as we could assume that once our make_request_fn()
> returns, any bios we "recursively" submitted against the local backend
> would be dispatched. Which used to be the case.

It'd be useful to know whether this patch fixes your issue:
https://patchwork.kernel.org/patch/7398411/

Ming Lei didn't like it due to concerns about I contexts changing
(whereby breaking merging that occurs via plugging).

But if it _does_ fix your issue then the case for the change is
increased; and we just need to focus on addressing Ming's concerns
(Mikulas has some ideas).

Conversely, and in parallel, Mikulas can look to see if your approach
fixes the observed dm-snapshot deadlock that he set out to fix.