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

[Hillf Danton]

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?

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