On Fri, 2013-08-09 at 18:26 +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.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo....@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);
> + if (!nrg) {
> + spin_unlock(&resv->lock);
> + nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> + if (!nrg) {
> + chg = -ENOMEM;
> + goto out;
Just return -ENOMEM here.
> + }
> + goto retry;
> + }
> + }
> +
You seem to be right, at least in my workloads, the hold times for the
region lock is quite small, so a spinlock is better than a sleeping
lock.
That said, this code is quite messy, but I cannot think of a
better/cleaner approach right now.
> 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;
--
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