mm, hugetlb: fix race in region tracking

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 only
mmap_sem (exclusively).  This doesn't prevent other tasks from modifying
the region structure, so it can be modified by two processes
concurrently.

To solve this, introduce a spinlock to resv_map and make region
manipulation function grab it before they do actual work.

[davidlohr@hp.com: updated changelog]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Suggested-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 20 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 454b311373c5..5a2515a774b5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -135,15 +135,8 @@ 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;
@@ -156,6 +149,7 @@ 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)
@@ -185,15 +179,18 @@ 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)
@@ -203,15 +200,21 @@ 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;
-                nrg->from = f;
-                nrg->to   = f;
-                INIT_LIST_HEAD(&nrg->link);
-                list_add(&nrg->link, rg->link.prev);
+                if (!nrg) {
+                        spin_unlock(&resv->lock);
+                        nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
+                        if (!nrg)
+                                return -ENOMEM;
+
+                        nrg->from = f;
+                        nrg->to   = f;
+                        INIT_LIST_HEAD(&nrg->link);
+                        goto retry;
+                }
 
-                return t - f;
+                list_add(&nrg->link, rg->link.prev);
+                chg = t - f;
+                goto out_nrg;
         }
 
         /* Round our left edge to the current segment if it encloses us. */
@@ -224,7 +227,7 @@ 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
@@ -235,6 +238,14 @@ static long region_chg(struct resv_map *resv, long f, long t)
                 }
                 chg -= rg->to - rg->from;
         }
+
+out:
+        spin_unlock(&resv->lock);
+        /*  We already know we raced and no longer need the new region */
+        kfree(nrg);
+        return chg;
+out_nrg:
+        spin_unlock(&resv->lock);
         return chg;
 }
 
@@ -244,12 +255,13 @@ 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) {
@@ -266,6 +278,9 @@ static long region_truncate(struct resv_map *resv, long end)
                 list_del(&rg->link);
                 kfree(rg);
         }
+
+out:
+        spin_unlock(&resv->lock);
         return chg;
 }
 
@@ -275,6 +290,7 @@ 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;
@@ -290,6 +306,7 @@ static long region_count(struct resv_map *resv, long f, long t)
 
                 chg += seg_to - seg_from;
         }
+        spin_unlock(&resv->lock);
 
         return chg;
 }
@@ -387,6 +404,7 @@ 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;