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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Mon, Jul 29, 2013 at 11:53:05AM -0700, Davidlohr Bueso wrote:
> On Mon, 2013-07-29 at 14:31 +0900, Joonsoo Kim 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.
> 
> I think we can use a rwsem here instead of a spinlock, similar to my
> previous 1/2 approach.

Hello, Davidlohr.

As I said in reply to Hillf, I prefer to use a spinlock.
Please refer that reply.

Thanks.

> 
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > 
> > 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
> > @@ -134,15 +134,8 @@ static inline struct hugepage_subpool *subpool_vma(struct vm_area_struct *vma)
> >   * Region tracking -- allows tracking of reservations and instantiated pages
> >   *                    across the pages in a mapping.
> >   *
> > - * The region data structures are protected by a combination of the mmap_sem
> > - * and the hugetlb_instantiation_mutex.  To access or modify a region the caller
> > - * must either hold the mmap_sem for write, or the mmap_sem for read and
> > - * the hugetlb_instantiation_mutex:
> > - *
> > - *	down_write(&mm->mmap_sem);
> > - * or
> > - *	down_read(&mm->mmap_sem);
> > - *	mutex_lock(&hugetlb_instantiation_mutex);
> > + * The region data structures are embedded into a resv_map and
> > + * protected by a resv_map's lock
> >   */
> >  struct file_region {
> >  	struct list_head link;
> > @@ -155,6 +148,7 @@ static long region_add(struct resv_map *resv, long f, long t)
> >  	struct list_head *head = &resv->regions;
> >  	struct file_region *rg, *nrg, *trg;
> >  
> > +	spin_lock(&resv->lock);
> >  	/* Locate the region we are either in or before. */
> >  	list_for_each_entry(rg, head, link)
> >  		if (f <= rg->to)
> > @@ -184,6 +178,7 @@ static long region_add(struct resv_map *resv, long f, long t)
> >  	}
> >  	nrg->from = f;
> >  	nrg->to = t;
> > +	spin_unlock(&resv->lock);
> >  	return 0;
> >  }
> >  
> > @@ -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);
> > -		if (!nrg)
> > -			return -ENOMEM;
> > +		if (!nrg) {
> > +			chg = -ENOMEM;
> > +			goto out;
> > +		}
> > +
> >  		nrg->from = f;
> >  		nrg->to   = f;
> >  		INIT_LIST_HEAD(&nrg->link);
> >  		list_add(&nrg->link, rg->link.prev);
> >  
> > -		return t - f;
> > +		chg = t - f;
> > +		goto out;
> >  	}
> >  
> >  	/* Round our left edge to the current segment if it encloses us. */
> > @@ -223,7 +223,7 @@ static long region_chg(struct resv_map *resv, long f, long t)
> >  		if (&rg->link == head)
> >  			break;
> >  		if (rg->from > t)
> > -			return chg;
> > +			goto out;
> >  
> >  		/* We overlap with this area, if it extends further than
> >  		 * us then we must extend ourselves.  Account for its
> > @@ -234,6 +234,9 @@ static long region_chg(struct resv_map *resv, long f, long t)
> >  		}
> >  		chg -= rg->to - rg->from;
> >  	}
> > +
> > +out:
> > +	spin_unlock(&resv->lock);
> >  	return chg;
> >  }
> >  
> > @@ -243,12 +246,13 @@ static long region_truncate(struct resv_map *resv, long end)
> >  	struct file_region *rg, *trg;
> >  	long chg = 0;
> >  
> > +	spin_lock(&resv->lock);
> >  	/* Locate the region we are either in or before. */
> >  	list_for_each_entry(rg, head, link)
> >  		if (end <= rg->to)
> >  			break;
> >  	if (&rg->link == head)
> > -		return 0;
> > +		goto out;
> >  
> >  	/* If we are in the middle of a region then adjust it. */
> >  	if (end > rg->from) {
> > @@ -265,6 +269,9 @@ static long region_truncate(struct resv_map *resv, long end)
> >  		list_del(&rg->link);
> >  		kfree(rg);
> >  	}
> > +
> > +out:
> > +	spin_unlock(&resv->lock);
> >  	return chg;
> >  }
> >  
> > @@ -274,6 +281,7 @@ static long region_count(struct resv_map *resv, long f, long t)
> >  	struct file_region *rg;
> >  	long chg = 0;
> >  
> > +	spin_lock(&resv->lock);
> >  	/* Locate each segment we overlap with, and count that overlap. */
> >  	list_for_each_entry(rg, head, link) {
> >  		long seg_from;
> > @@ -289,6 +297,7 @@ static long region_count(struct resv_map *resv, long f, long t)
> >  
> >  		chg += seg_to - seg_from;
> >  	}
> > +	spin_unlock(&resv->lock);
> >  
> >  	return chg;
> >  }
> > @@ -386,6 +395,7 @@ struct resv_map *resv_map_alloc(void)
> >  		return NULL;
> >  
> >  	kref_init(&resv_map->refs);
> > +	spin_lock_init(&resv_map->lock);
> >  	INIT_LIST_HEAD(&resv_map->regions);
> >  
> >  	return resv_map;
> 
> 
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