Re: [PATCH v2 09/20] mm, hugetlb: protect region tracking via newly introduced resv_map lock

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Joonsoo Kim <iamjoonsoo.kim@lge.com> writes:

> 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@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 d9cabf6..73034dd 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)
> @@ -184,15 +178,18 @@ static long region_add(struct resv_map *resv, long f, long t)
>  	}
>  	nrg->from = f;
>  	nrg->to = t;
> +	spin_unlock(&resv->lock);
>  	return 0;
>  }
>
>  static long region_chg(struct resv_map *resv, long f, long t)
>  {
>  	struct list_head *head = &resv->regions;
> -	struct file_region *rg, *nrg;
> +	struct file_region *rg, *nrg = NULL;
>  	long chg = 0;
>
> +retry:
> +	spin_lock(&resv->lock);
>  	/* Locate the region we are before or in. */
>  	list_for_each_entry(rg, head, link)
>  		if (f <= rg->to)
> @@ -202,15 +199,27 @@ 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) {
> +			nrg = kmalloc(sizeof(*nrg), GFP_NOWAIT);

Do we really need to have the GFP_NOWAIT allocation attempt. Why can't we simply say
allocate and retry ? Or should resv->lock be a mutex ?

> +			if (!nrg) {
> +				spin_unlock(&resv->lock);
> +				nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> +				if (!nrg) {
> +					chg = -ENOMEM;
> +					goto out;
> +				}
> +				goto retry;
> +			}
> +		}
> +
>  		nrg->from = f;
>  		nrg->to   = f;
>  		INIT_LIST_HEAD(&nrg->link);
>  		list_add(&nrg->link, rg->link.prev);
> +		nrg = NULL;
>
> -		return t - f;
> +		chg = t - f;
> +		goto out_locked;
>  	}
>
>  	/* Round our left edge to the current segment if it encloses us. */
> @@ -223,7 +232,7 @@ 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_locked;
>
>  		/* We overlap with this area, if it extends further than
>  		 * us then we must extend ourselves.  Account for its
> @@ -234,6 +243,11 @@ static long region_chg(struct resv_map *resv, long f, long t)
>  		}
>  		chg -= rg->to - rg->from;
>  	}
> +
> +out_locked:
> +	spin_unlock(&resv->lock);
> +out:
> +	kfree(nrg);
>  	return chg;
>  }
>
> @@ -243,12 +257,13 @@ 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 +280,9 @@ 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 +292,7 @@ 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 +308,7 @@ 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 +406,7 @@ 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;
> -- 
> 1.7.9.5

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