[PATCH 1/2] hugepage: protect file regions with rwsem

[Davidlohr Bueso]

The next patch will introduce parallel hugepage fault paths, replacing
the global hugetbl_instantiation_mutex with a table of hashed mutexes.
In order to prevent races, introduce a rw-semaphore that exclusively
serializes access to file region tracking structure.

Thanks to Konstantin Khlebnikov and Joonsoo Kim for pointing this issue out.

Signed-off-by: Davidlohr Bueso <davidlohr.bu...@hp.com>
---
 mm/hugetlb.c | 47 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 83aff0a..4c3f4f0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -134,16 +134,12 @@ 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_instantion_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);
+ * With the parallelization of the hugepage fault path, the region_rwsem 
replaces
+ * the original hugetlb_instantiation_mutex, serializing access to the chains 
of
+ * file regions.
  */
+DECLARE_RWSEM(region_rwsem);
+
 struct file_region {
        struct list_head link;
        long from;
@@ -154,6 +150,8 @@ static long region_add(struct list_head *head, long f, long 
t)
 {
        struct file_region *rg, *nrg, *trg;
 
+       down_write(&region_rwsem);
+
        /* Locate the region we are either in or before. */
        list_for_each_entry(rg, head, link)
                if (f <= rg->to)
@@ -183,6 +181,8 @@ static long region_add(struct list_head *head, long f, long 
t)
        }
        nrg->from = f;
        nrg->to = t;
+
+       up_write(&region_rwsem);
        return 0;
 }
 
@@ -191,6 +191,8 @@ static long region_chg(struct list_head *head, long f, long 
t)
        struct file_region *rg, *nrg;
        long chg = 0;
 
+       down_write(&region_rwsem);
+
        /* Locate the region we are before or in. */
        list_for_each_entry(rg, head, link)
                if (f <= rg->to)
@@ -201,17 +203,21 @@ static long region_chg(struct list_head *head, 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 done_write;
+               }
                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 done_write;
        }
 
        /* Round our left edge to the current segment if it encloses us. */
+       downgrade_write(&region_rwsem);
        if (f > rg->from)
                f = rg->from;
        chg = t - f;
@@ -221,7 +227,7 @@ static long region_chg(struct list_head *head, long f, long 
t)
                if (&rg->link == head)
                        break;
                if (rg->from > t)
-                       return chg;
+                       break;
 
                /* We overlap with this area, if it extends further than
                 * us then we must extend ourselves.  Account for its
@@ -232,6 +238,11 @@ static long region_chg(struct list_head *head, long f, 
long t)
                }
                chg -= rg->to - rg->from;
        }
+
+       up_read(&region_rwsem);
+       return chg;
+done_write:
+       up_write(&region_rwsem);
        return chg;
 }
 
@@ -240,12 +251,14 @@ static long region_truncate(struct list_head *head, long 
end)
        struct file_region *rg, *trg;
        long chg = 0;
 
+       down_write(&region_rwsem);
+
        /* 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 done;
 
        /* If we are in the middle of a region then adjust it. */
        if (end > rg->from) {
@@ -262,6 +275,9 @@ static long region_truncate(struct list_head *head, long 
end)
                list_del(&rg->link);
                kfree(rg);
        }
+
+done:
+       up_write(&region_rwsem);
        return chg;
 }
 
@@ -270,6 +286,8 @@ static long region_count(struct list_head *head, long f, 
long t)
        struct file_region *rg;
        long chg = 0;
 
+       down_read(&region_rwsem);
+
        /* Locate each segment we overlap with, and count that overlap. */
        list_for_each_entry(rg, head, link) {
                long seg_from;
@@ -286,6 +304,7 @@ static long region_count(struct list_head *head, long f, 
long t)
                chg += seg_to - seg_from;
        }
 
+       up_read(&region_rwsem);
        return chg;
 }
 
-- 
1.7.11.7

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