Re: [PATCH v2] kmemleak: survive in a low-memory situation

[Michal Hocko <mhocko@kernel.org>]

On Thu 03-01-19 11:51:57, Qian Cai wrote:
> On 1/3/19 4:32 AM, Michal Hocko wrote:
> > On Wed 02-01-19 13:06:19, Qian Cai wrote:
> > [...]
> >> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> >> index f9d9dc250428..9e1aa3b7df75 100644
> >> --- a/mm/kmemleak.c
> >> +++ b/mm/kmemleak.c
> >> @@ -576,6 +576,16 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
> >>  	struct rb_node **link, *rb_parent;
> >>  
> >>  	object = kmem_cache_alloc(object_cache, gfp_kmemleak_mask(gfp));
> >> +#ifdef CONFIG_PREEMPT_COUNT
> >> +	if (!object) {
> >> +		/* last-ditch effort in a low-memory situation */
> >> +		if (irqs_disabled() || is_idle_task(current) || in_atomic())
> >> +			gfp = GFP_ATOMIC;
> >> +		else
> >> +			gfp = gfp_kmemleak_mask(gfp) | __GFP_DIRECT_RECLAIM;
> >> +		object = kmem_cache_alloc(object_cache, gfp);
> >> +	}
> >> +#endif
> > 
> > I do not get it. How can this possibly help when gfp_kmemleak_mask()
> > adds __GFP_NOFAIL modifier to the given gfp mask? Or is this not the
> > case anymore in some tree?
> 
> Well, __GFP_NOFAIL can still fail easily without __GFP_DIRECT_RECLAIM in a
> low-memory situation.

OK, I guess I understand now. So the issue is that a (general) atomic
allocation will provide its gfp mask down to kmemleak and you are
trying/hoping that if the allocation is no from an atomic context then
you can fortify it by using a sleepable allocation for the kmemleak
metadata or giving it access to memory reserves for atomic allocations.

I think this is still fragile because most atomic allocations are for a
good reason. As I've said earlier the current implementation which
abuses __GFP_NOFAIL is fra from great and we have discussed some
alternatives. Not sure whan came out of it.

I will not object to this workaround but I strongly believe that
kmemleak should rethink the metadata allocation strategy to be really
robust.
-- 
Michal Hocko
SUSE Labs