[Xen-devel] [PATCH 2/2] x86/p2m: adjust non-PoD accounting in p2m_pod_decrease_reservation()

[Jan Beulich <jbeulich@xxxxxxxx>]

Throughout the function the equation

        pod + nonpod == (1UL << order)

should hold. This has been violated by the final loop of the function:
* changing a range from a type other than p2m_populate_on_demand to
  p2m_invalid doesn't alter the amount of non-PoD pages in the region,
* changing a range from p2m_populate_on_demand to p2m_invalid does
  increase the amount of non-PoD pages in the region along with
  decreasing the amount of PoD pages there.
Fortunately the variable isn't used anymore after the loop. Instead of
correcting the updating of the "nonpod" variable, however, drop it
altogether, to avoid getting the above equation to not hold again by a
future change.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -509,7 +509,7 @@ p2m_pod_decrease_reservation(struct doma
     unsigned long ret = 0, i, n;
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
     bool_t steal_for_cache;
-    long pod, nonpod, ram;
+    long pod = 0, ram = 0;
 
     gfn_lock(p2m, gfn, order);
     pod_lock(p2m);
@@ -524,8 +524,6 @@ p2m_pod_decrease_reservation(struct doma
     if ( unlikely(d->is_dying) )
         goto out_unlock;
 
-    pod = nonpod = ram = 0;
-
     /* Figure out if we need to steal some freed memory for our cache */
     steal_for_cache =  ( p2m->pod.entry_count > p2m->pod.count );
 
@@ -539,19 +537,15 @@ p2m_pod_decrease_reservation(struct doma
         n = 1UL << min(order, cur_order);
         if ( t == p2m_populate_on_demand )
             pod += n;
-        else
-        {
-            nonpod += n;
-            if ( p2m_is_ram(t) )
-                ram += n;
-        }
+        else if ( p2m_is_ram(t) )
+            ram += n;
     }
 
     /* No populate-on-demand?  Don't need to steal anything?  Then we're 
done!*/
     if ( !pod && !steal_for_cache )
         goto out_unlock;
 
-    if ( !nonpod )
+    if ( i == pod )
     {
         /*
          * All PoD: Mark the whole region invalid and tell caller
@@ -587,7 +581,7 @@ p2m_pod_decrease_reservation(struct doma
          p2m_pod_zero_check_superpage(p2m, _gfn(gfn_x(gfn) & ~(SUPERPAGE_PAGES 
- 1))) )
     {
         pod = 1UL << order;
-        ram = nonpod = 0;
+        ram = 0;
         ASSERT(steal_for_cache == (p2m->pod.entry_count > p2m->pod.count));
     }
 
@@ -655,7 +649,6 @@ p2m_pod_decrease_reservation(struct doma
 
             steal_for_cache =  ( p2m->pod.entry_count > p2m->pod.count );
 
-            nonpod -= n;
             ram -= n;
             ret += n;
         }


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