Re: block: fix blk_queue_split() resource exhaustion

[Mike Snitzer]

On Thu, Jul 07 2016 at  1:35am -0400,
NeilBrown <neilb@xxxxxxxx> wrote:

> On Wed, Jun 22 2016, Lars Ellenberg wrote:
> 
> > For a long time, generic_make_request() converts recursion into
> > iteration by queuing recursive arguments on current->bio_list.
> >
> > This is convenient for stacking drivers,
> > the top-most driver would take the originally submitted bio,
> > and re-submit a re-mapped version of it, or one or more clones,
> > or one or more new allocated bios to its backend(s). Which
> > are then simply processed in turn, and each can again queue
> > more "backend-bios" until we reach the bottom of the driver stack,
> > and actually dispatch to the real backend device.
> >
> > Any stacking driver ->make_request_fn() could expect that,
> > once it returns, any backend-bios it submitted via recursive calls
> > to generic_make_request() would now be processed and dispatched, before
> > the current task would call into this driver again.
> >
> > This is changed by commit
> >   54efd50 block: make generic_make_request handle arbitrarily sized bios
> >
> > Drivers may call blk_queue_split() inside their ->make_request_fn(),
> > which may split the current bio into a front-part to be dealt with
> > immediately, and a remainder-part, which may need to be split even
> > further. That remainder-part will simply also be pushed to
> > current->bio_list, and would end up being head-of-queue, in front
> > of any backend-bios the current make_request_fn() might submit during
> > processing of the fron-part.
> >
> > Which means the current task would immediately end up back in the same
> > make_request_fn() of the same driver again, before any of its backend
> > bios have even been processed.
> >
> > This can lead to resource starvation deadlock.
> > Drivers could avoid this by learning to not need blk_queue_split(),
> > or by submitting their backend bios in a different context (dedicated
> > kernel thread, work_queue context, ...). Or by playing funny re-ordering
> > games with entries on current->bio_list.
> >
> > Instead, I suggest to distinguish between recursive calls to
> > generic_make_request(), and pushing back the remainder part in
> > blk_queue_split(), by pointing current->bio_lists to a
> > 	struct recursion_to_iteration_bio_lists {
> > 		struct bio_list recursion;
> > 		struct bio_list remainder;
> > 	}
> >
> > To have all bios targeted to drivers lower in the stack processed before
> > processing the next piece of a bio targeted at the higher levels,
> > as long as queued bios resulting from recursion are available,
> > they will continue to be processed in FIFO order.
> > Pushed back bio-parts resulting from blk_queue_split() will be processed
> > in LIFO order, one-by-one, whenever the recursion list becomes empty.
> 
> I really like this change.  It seems to precisely address the problem.
> The "problem" being that requests for "this" device are potentially
> mixed up with requests from underlying devices.
> However I'm not sure it is quite general enough.
> 
> The "remainder" list is a stack of requests aimed at "this" level or
> higher, and I think it will always exactly fit that description.
> The "recursion" list needs to be a queue of requests aimed at the next
> level down, and that doesn't quiet work, because once you start acting
> on the first entry in that list, all the rest become "this" level.
> 
> I think you can address this by always calling ->make_request_fn with an
> empty "recursion", then after the call completes, splice the "recursion"
> list that resulted (if any) on top of the "remainder" stack.
> 
> This way, the "remainder" stack is always "requests for lower-level
> devices before request for upper level devices" and the "recursion"
> queue is always "requests for devices below the current level".
> 
> I also really *don't* like the idea of punting to a separate thread

Hi Neil,

Was this concern about "punting to a separate thread" in reference to
the line of work from Mikulas at the top of this 'wip' branch?
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip

> - it seems to be just delaying the problem.

Have you looked at this closely?  Not seeing how you can say that given
that on schedule the bios on current->bio_list are flushed.

The incremental work to delay the offload of queued bios is just meant
to preserve existing bio submission order unless there is reason to
believe a deadlock exists.

I would agree that this timer based approach is rather "gross" to some
degree _but_ it beats deadlocks!  This code needs fixing.  And the fix
cannot be constrained to bio_queue_split() because DM isn't even using
it.

Mike