Xfs lockdep warning with for-dave-for-4.6 branch

[Michal Hocko]

On Fri 03-06-16 09:22:54, Dave Chinner wrote:
> On Thu, Jun 02, 2016 at 05:46:19PM +0200, Michal Hocko wrote:
> > On Thu 02-06-16 17:11:16, Peter Zijlstra wrote:
> > > With scope I mostly meant the fact that you have two calls that you need
> > > to pair up. That's not really nice as you can 'annotate' a _lot_ of code
> > > in between. I prefer the narrower annotations where you annotate a
> > > single specific site.
> > 
> > Yes, I can see you point. What I meant to say is that we would most
> > probably end up with the following pattern
> > 	lockdep_trace_alloc_enable()
> > 	some_foo_with_alloc(gfp_mask);
> > 	lockdep_trace_alloc_disable()
> >
> > and some_foo_with_alloc might be a lot of code.
> 
> That's the problem I see with this - the only way to make it
> maintainable is to precede each enable/disable() pair with a comment
> explaining *exactly* what those calls are protecting.  And that, in
> itself, becomes a maintenance problem, because then code several
> layers deep has no idea what context it is being called from and we
> are likely to disable warnings in contexts where we probably
> shouldn't be.

I am not sure I understand what you mean here. I thought the problem is
that:

func_A (!trans. context)		func_B (trans. context)
 foo1()					  foo2()
   bar(inode, GFP_KERNEL)		    bar(inode, GFP_NOFS)

so bar(inode, gfp) can be called from two different contexts which
would confuse the lockdep. And the workaround would be annotating bar
depending on the context it is called from - either pass a special gfp
flag or do disable/enable thing. In both cases that anotation should be
global for the whole func_A, no? Or is it possible that something in
that path would really need a reclaim lockdep detection?

> I think such an annotation approach really requires per-alloc site
> annotation, the reason for it should be more obvious from the
> context. e.g. any function that does memory alloc and takes an
> optional transaction context needs annotation. Hence, from an XFS
> perspective, I think it makes more sense to add a new KM_ flag to
> indicate this call site requirement, then jump through whatever
> lockdep hoop is required within the kmem_* allocation wrappers.
> e.g, we can ignore the new KM_* flag if we are in a transaction
> context and so the flag is only activated in the situations were
> we currently enforce an external GFP_NOFS context from the call
> site.....

Hmm, I thought we would achive this by using the scope GFP_NOFS usage
which would mark those transaction related conctexts and no lockdep
specific workarounds would be needed...

-- 
Michal Hocko
SUSE Labs