Re: Kmemleak infrastructure improvement for task_struct leaks and call_rcu()

[Paul E. McKenney]

On Thu, May 07, 2020 at 06:14:19PM +0100, Catalin Marinas wrote:
> On Wed, May 06, 2020 at 10:40:19AM -0700, Paul E. McKenney wrote:
> > On Wed, May 06, 2020 at 12:22:37PM -0400, Qian Cai wrote:
> > > == call_rcu() leaks ==
> > > Another issue that might be relevant is that it seems sometimes,
> > > kmemleak will give a lot of false positives (hundreds) because the
> > > memory was supposed to be freed by call_rcu()  (for example, in
> > > dst_release()) but for some reasons, it takes a long time probably
> > > waiting for grace periods or some kind of RCU self-stall, but the
> > > memory had already became an orphan. I am not sure how we are going
> > > to resolve this properly until we have to figure out why call_rcu()
> > > is taking so long to finish?
> > 
> > I know nothing about kmemleak, but I won't let that stop me from making
> > random suggestions...
> > 
> > One approach is to do an rcu_barrier() inside kmemleak just before
> > printing leaked blocks, and check to see if any are still leaked after
> > the rcu_barrier().
> 
> The main issue is that kmemleak doesn't stop the world when scanning
> (which can take over a minute, depending on your hardware), so we get
> lots of transient pointer misses. There are some heuristics but
> obviously they don't always work.
> 
> With RCU, objects are queued for RCU freeing later and chained via
> rcu_head.next (IIUC). Under load, this list can be pretty volatile and
> if this happen during kmemleak scanning, it's sufficient to lose track
> of a next pointer and the rest of the list would be reported as a leak.
> 
> I think rcu_barrier() just before the starting the kmemleak scanning may
> help if it reduces the number of objects queued.

It might, especially if the call_rcu() rate is lower after the
rcu_barrier() than it was beforehand.  Which might well be the case when
a large cleanup activity ended just before rcu_barrier() was invoked.

> Now, I wonder whether kmemleak itself can break this RCU chain. The
> kmemleak metadata is allocated on a slab alloc callback. The freeing,
> however, is done using call_rcu() because originally calling back into
> the slab freeing from kmemleak_free() didn't go well. Since the
> kmemleak_object structure is not tracked by kmemleak, I wonder whether
> its rcu_head would break this directed pointer reference graph.

It is true that kmemleak could decide that being passed to call_rcu()
as being freed.  However, it would need to know the rcu_head offset.
And there are (or were) a few places that pass linked structures to
call_rcu(), and kmemleak would presumably need to mark them all free
at that point.  Or maybe accept the much lower false-positive rate from
not marking them.

> Let's try the rcu_barrier() first and I'll think about the metadata case
> over the weekend.

Looking forward to hearing how it goes!

							Thanx, Paul