hugetlb: region_chg provides only cache entry

Current behavior is that region_chg provides both a cache entry in
resv->region_cache, AND a placeholder entry in resv->regions.
region_add first tries to use the placeholder, and if it finds that the
placeholder has been deleted by a racing region_del call, it uses the
cache entry.

This behavior is completely unnecessary and is removed in this patch for
a couple of reasons:

1. region_add needs to either find a cached file_region entry in
   resv->region_cache, or find an entry in resv->regions to expand. It
   does not need both.

2. region_chg adding a placeholder entry in resv->regions opens up
   a possible race with region_del, where region_chg adds a placeholder
   region in resv->regions, and this region is deleted by a racing call
   to region_del during region_chg execution or before region_add is
   called. Removing the race makes the code easier to reason about and
   maintain.

In addition, a follow up patch in another series that disables region
coalescing, which would be further complicated if the race with
region_del exists.

Link: http://lkml.kernel.org/r/20190919200428.188797-2-almasrymina@google.com
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 18c92cb..17178db 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -246,14 +246,10 @@ struct file_region {
 
 /*
  * Add the huge page range represented by [f, t) to the reserve
- * map.  In the normal case, existing regions will be expanded
- * to accommodate the specified range.  Sufficient regions should
- * exist for expansion due to the previous call to region_chg
- * with the same range.  However, it is possible that region_del
- * could have been called after region_chg and modifed the map
- * in such a way that no region exists to be expanded.  In this
- * case, pull a region descriptor from the cache associated with
- * the map and use that for the new range.
+ * map.  Existing regions will be expanded to accommodate the specified
+ * range, or a region will be taken from the cache.  Sufficient regions
+ * must exist in the cache due to the previous call to region_chg with
+ * the same range.
  *
  * Return the number of new huge pages added to the map.  This
  * number is greater than or equal to zero.
@@ -272,9 +268,8 @@ static long region_add(struct resv_map *resv, long f, long t)
 
        /*
         * If no region exists which can be expanded to include the
-        * specified range, the list must have been modified by an
-        * interleving call to region_del().  Pull a region descriptor
-        * from the cache and use it for this range.
+        * specified range, pull a region descriptor from the cache
+        * and use it for this range.
         */
        if (&rg->link == head || t < rg->from) {
                VM_BUG_ON(resv->region_cache_count <= 0);
@@ -339,15 +334,9 @@ out_locked:
  * call to region_add that will actually modify the reserve
  * map to add the specified range [f, t).  region_chg does
  * not change the number of huge pages represented by the
- * map.  However, if the existing regions in the map can not
- * be expanded to represent the new range, a new file_region
- * structure is added to the map as a placeholder.  This is
- * so that the subsequent region_add call will have all the
- * regions it needs and will not fail.
- *
- * Upon entry, region_chg will also examine the cache of region descriptors
- * associated with the map.  If there are not enough descriptors cached, one
- * will be allocated for the in progress add operation.
+ * map.  A new file_region structure is added to the cache
+ * as a placeholder, so that the subsequent region_add
+ * call will have all the regions it needs and will not fail.
  *
  * Returns the number of huge pages that need to be added to the existing
  * reservation map for the range [f, t).  This number is greater or equal to
@@ -357,10 +346,9 @@ out_locked:
 static long region_chg(struct resv_map *resv, long f, long t)
 {
        struct list_head *head = &resv->regions;
-       struct file_region *rg, *nrg = NULL;
+       struct file_region *rg;
        long chg = 0;
 
-retry:
        spin_lock(&resv->lock);
 retry_locked:
        resv->adds_in_progress++;
@@ -378,10 +366,8 @@ retry_locked:
                spin_unlock(&resv->lock);
 
                trg = kmalloc(sizeof(*trg), GFP_KERNEL);
-               if (!trg) {
-                       kfree(nrg);
+               if (!trg)
                        return -ENOMEM;
-               }
 
                spin_lock(&resv->lock);
                list_add(&trg->link, &resv->region_cache);
@@ -394,28 +380,6 @@ retry_locked:
                if (f <= rg->to)
                        break;
 
-       /* If we are below the current region then a new region is required.
-        * 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) {
-               if (!nrg) {
-                       resv->adds_in_progress--;
-                       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;
-               }
-
-               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. */
        if (f > rg->from)
                f = rg->from;
@@ -440,11 +404,6 @@ retry_locked:
 
 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;
 }