Re: [PATCH v3 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug

[Joel Fernandes]

On Tue, Sep 05, 2023 at 08:09:16AM +0100, Lorenzo Stoakes wrote:
> On Mon, Sep 04, 2023 at 06:08:04PM +0000, Joel Fernandes (Google) wrote:
> > It is unsafe to dump vmalloc area information when trying to do so from
> > some contexts. Add a safer trylock version of the same function to do a
> > best-effort VMA finding and use it from vmalloc_dump_obj().
> 
> It'd be nice to have more details as to precisely which contexts and what this
> resolves.

True. I was hoping the 'trylock' mention would be sufficient (example hardirq
context interrupting a lock-held region) but you're right.

> > [applied test robot feedback on unused function fix.]
> > [applied Uladzislau feedback on locking.]
> >
> > Reported-by: Zhen Lei <thunder.leizhen@xxxxxxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: rcu@xxxxxxxxxxxxxxx
> > Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > Fixes: 98f180837a89 ("mm: Make mem_dump_obj() handle vmalloc() memory")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  mm/vmalloc.c | 26 ++++++++++++++++++++++----
> >  1 file changed, 22 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 93cf99aba335..2c6a0e2ff404 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> >  #ifdef CONFIG_PRINTK
> >  bool vmalloc_dump_obj(void *object)
> >  {
> > -	struct vm_struct *vm;
> >  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > +	const void *caller;
> > +	struct vm_struct *vm;
> > +	struct vmap_area *va;
> > +	unsigned long addr;
> > +	unsigned int nr_pages;
> >
> > -	vm = find_vm_area(objp);
> > -	if (!vm)
> > +	if (!spin_trylock(&vmap_area_lock))
> > +		return false;
> 
> It'd be good to have a comment here explaining why we must trylock here. I am
> also concerned that in the past this function would return false only if the
> address was not a vmalloc one, but now it might just return false due to lock
> contention and the user has no idea which it is?
> 
> I'd want to at least output "vmalloc region cannot lookup lock contention"
> vs. the below cannot find case.

In the patch 2/2 we do print if the address looks like a vmalloc address even
if the vmalloc look up fails.

Also the reporter's usecase is not a common one. We only attempt to dump
information if there was a debug objects failure (example if somebody did a
double call_rcu). In such a situation, the patch will prevent a deadlock and
still print something about the address.

> Under heavy lock contention aren't you potentially breaking the ability to
> introspect vmalloc addresses? Wouldn't it be better to explicitly detect the
> contexts under which acquiring this spinlock is not appropriate?

Yes this is a good point, but there's another case as well: PREEMPT_RT can
sleep on lock contention (as spinlocks are sleeping) and we can't sleep from
call_rcu() as it may be called in contexts that cannot sleep. So we handle
that also using trylock.

Thanks for the review!

 - Joel


> 
> > +	va = __find_vmap_area((unsigned long)objp, &vmap_area_root);
> > +	if (!va) {
> > +		spin_unlock(&vmap_area_lock);
> >  		return false;
> > +	}
> > +
> > +	vm = va->vm;
> > +	if (!vm) {
> > +		spin_unlock(&vmap_area_lock);
> > +		return false;
> > +	}
> > +	addr = (unsigned long)vm->addr;
> > +	caller = vm->caller;
> > +	nr_pages = vm->nr_pages;
> > +	spin_unlock(&vmap_area_lock);
> >  	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> > -		vm->nr_pages, (unsigned long)vm->addr, vm->caller);
> > +		nr_pages, addr, caller);
> >  	return true;
> >  }
> >  #endif
> > --
> > 2.42.0.283.g2d96d420d3-goog
> >