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....@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 <da...@gibson.dropbear.id.au>
> > > > > Signed-off-by: Joonsoo Kim <iamjoonsoo....@lge.com>
> > > > > [Updated changelog]
> > > > > Signed-off-by: Davidlohr Bueso <davidl...@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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