Re: [PATCH 3/8] mm, hugetlb: fix race in region tracking

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Jan 27, 2014 at 06:34:17PM -0800, Davidlohr Bueso wrote:
> On Mon, 2014-01-27 at 20:53 -0500, Naoya Horiguchi wrote:
> > Hi Davidlohr,
> > 
> > On Mon, Jan 27, 2014 at 01:44:02PM -0800, Davidlohr Bueso wrote:
> > > On Mon, 2014-01-27 at 16:02 -0500, Naoya Horiguchi wrote:
> > > > On Sun, Jan 26, 2014 at 07:52:21PM -0800, Davidlohr Bueso wrote:
> > > > > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > > > 
> > > > > There is a race condition if we map a same file on different processes.
> > > > > Region tracking is protected by mmap_sem and hugetlb_instantiation_mutex.
> > > > > When we do mmap, we don't grab a hugetlb_instantiation_mutex, but only the,
> > > > > mmap_sem (exclusively). This doesn't prevent other tasks from modifying the
> > > > > region structure, so it can be modified by two processes concurrently.
> > > > > 
> > > > > To solve this, introduce a spinlock to resv_map and make region manipulation
> > > > > function grab it before they do actual work.
> > > > > 
> > > > > Acked-by: David Gibson <david@gibson.dropbear.id.au>
> > > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > > > [Updated changelog]
> > > > > Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> > > > > ---
> > > > ...
> > > > > @@ -203,15 +200,23 @@ static long region_chg(struct resv_map *resv, long f, long t)
> > > > >  	 * Subtle, allocate a new region at the position but make it zero
> > > > >  	 * size such that we can guarantee to record the reservation. */
> > > > >  	if (&rg->link == head || t < rg->from) {
> > > > > -		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> > > > > -		if (!nrg)
> > > > > -			return -ENOMEM;
> > > > > +		if (!nrg) {
> > > > > +			spin_unlock(&resv->lock);
> > > > 
> > > > I think that doing kmalloc() inside the lock is simpler.
> > > > Why do you unlock and retry here?
> > > 
> > > This is a spinlock, no can do -- we've previously debated this and since
> > > the critical region is quite small, a non blocking lock is better suited
> > > here. We do the retry so we don't race once the new region is allocated
> > > after the lock is dropped.
> > 
> > Using spinlock instead of rw_sem makes sense.
> > But I'm not sure how the retry is essential to fix the race.
> > (Sorry I can't find the discussion log about this.)
> > As you did in your ver.1 (https://lkml.org/lkml/2013/7/26/296),
> > simply doing like below seems to be fine to me, is it right?
> > 
> >         if (&rg->link == head || t < rg->from) {
> > 		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> > 		if (!nrg) {
> > 			chg = -ENOMEM;
> > 			goto out_locked;
> > 		}
> > 		nrg->from = f;
> > 		...
> > 	}
> 
> That's nice and simple because we were using the rwsem version.
> 
> > 
> > In the current version nrg is initialized to NULL, so we always do retry
> > once when adding new file_region. That's not optimal to me.
> 
> Right, the retry can only occur once.
> 
> > 
> > If this retry is really essential for the fix, please comment the reason
> > both in patch description and inline comment. It's very important for
> > future code maintenance.
> 
> So we locate the corresponding region in the reserve map, and if we are
> below the current region, then we allocate a new one. Since we dropped
> the lock to allocate memory, we have to make sure that we still need the
> new region and that we don't race with the new status of the reservation
> map. This is the whole point of the retry, and I don't see it being
> suboptimal.

I'm afraid that you don't explain why you need drop the lock for memory
allocation. Are you saying that this unlocking comes from the difference
between rwsem and spin lock?

I think if we call kmalloc() with the lock held we don't have to check
that "we still need the new region" because resv->lock guarantees that
no other thread changes the reservation map, right?

> We just cannot retake the lock after we get the new region and just add
> it to to the list.
> 
> > 
> > And I noticed another point. I don't think the name of new goto label
> > 'out_locked' is a good one. 'out_unlock' or 'unlock' is better.
> 
> What worries me more is that we're actually freeing a valid new region
> (nrg) upon exit. We certainly don't do so in the current code, and it
> doesn't seem to be a leak. Instead, we should be doing:

You're right. There is another goto in region_chg() where we never do
the kmalloc, so calling kfree is a bug.

Thanks,
Naoya Horiguchi

> 	if (&rg->link == head || t < rg->from) {
> 		if (!nrg) {
> 			spin_unlock(&resv->lock);
> 			nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> 			if (!nrg)
> 				return -ENOMEM;
> 
> 			goto retry;
> 		}
> 
> 		nrg->from = f;
> 		nrg->to   = f;
> 		INIT_LIST_HEAD(&nrg->link);
> 		list_add(&nrg->link, rg->link.prev);
> 
> 		chg = t - f;
> 		goto out;
> 	}
> ...
> out:
> 	spin_unlock(&resv->lock);
> 	return chg;
> 
> 
> Thanks,
> Davidlohr
> 

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