Re: [PATCH 05/18] mm, hugetlb: protect region tracking via newlyintroduced resv_map lock

[Joonsoo Kim]

On Mon, Jul 29, 2013 at 04:58:57PM +0800, Hillf Danton wrote:
> On Mon, Jul 29, 2013 at 1:31 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> > 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,
> > grab a mmap_sem. This doesn't prevent other process to modify region
> > structure, so it can be modified by two processes concurrently.
> >
> > To solve this, I introduce a lock to resv_map and make region manipulation
> > function grab a lock before they do actual work. This makes region
> > tracking safe.
> >
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 2677c07..e29e28f 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -26,6 +26,7 @@ struct hugepage_subpool {
> >
> >  struct resv_map {
> >         struct kref refs;
> > +       spinlock_t lock;
> >         struct list_head regions;
> >  };
> >  extern struct resv_map *resv_map_alloc(void);
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 24c0111..bf2ee11 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> [...]
> > @@ -193,6 +188,7 @@ static long region_chg(struct resv_map *resv, long f, long t)
> >         struct file_region *rg, *nrg;
> >         long chg = 0;
> >
> > +       spin_lock(&resv->lock);
> >         /* Locate the region we are before or in. */
> >         list_for_each_entry(rg, head, link)
> >                 if (f <= rg->to)
> > @@ -203,14 +199,18 @@ static long region_chg(struct resv_map *resv, long f, long t)
> >          * size such that we can guarantee to record the reservation. */
> >         if (&rg->link == head || t < rg->from) {
> >                 nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> 
> Hm, you are allocating a piece of memory with spin lock held.
> How about replacing that spin lock with a mutex?

I think that lock held period here is very short, so mutex is not appropriate
to use. How about trying allocate with GFP_NOWAIT? And then if failed,
release the lock and allocate with GFP_KERNEL and retry at the beginnig.

Thanks.

> 
> > -               if (!nrg)
> > -                       return -ENOMEM;
> > +               if (!nrg) {
> > +                       chg = -ENOMEM;
> > +                       goto out;
> > +               }
> > +
> 
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