[PATCH 1 of 5] Allow decrease_reservation to be preempted if remove_page returns negative

[Andres Lagar-Cavilla <andres@lagarcavilla.org>]

 xen/common/memory.c |  29 ++++++++++++++++++++++++++++-
 1 files changed, 28 insertions(+), 1 deletions(-)


This will allow for handling of full paging rings in a more graceful manner.
ATM, remove_page does not return negative values, so this code is not yet
exercised.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

diff -r 15da109b0c7d -r 8d67f4d5bafa xen/common/memory.c
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -226,6 +226,8 @@ static void decrease_reservation(struct 
 
     for ( i = a->nr_done; i < a->nr_extents; i++ )
     {
+        int one_remove_succeeded = 0;
+
         if ( hypercall_preempt_check() )
         {
             a->preempted = 1;
@@ -255,8 +257,33 @@ static void decrease_reservation(struct 
             continue;
 
         for ( j = 0; j < (1 << a->extent_order); j++ )
-            if ( !guest_remove_page(a->domain, gmfn + j) )
+        {
+            int rv = guest_remove_page(a->domain, gmfn + j);
+            /* negative rv is a result of a mem event ring full
+             * in the presence of paging. We preempt the hypercall */
+            if ( rv < 0 )
+            {
+                /* Indicate we're done with this extent */
+                if ((j+1) == (1 << a->extent_order))
+                    i++;
+                a->preempted = 1;
+                raise_softirq(SCHEDULE_SOFTIRQ);
                 goto out;
+            }
+            /* rv of zero means we tried to remove a gfn with no backing
+             * mfn. It can be a bad argument, or, a continuation in the midst
+             * of an extent. Heuristically determine second case. */
+            if ( !rv )
+            {
+                /* Has to be first extent */
+                if ( i != a->nr_done )
+                    goto out;
+                /* No previous remove in this extent must have succeeded */
+                if ( one_remove_succeeded )
+                    goto out;
+            } else
+                one_remove_succeeded = 1;
+        }
     }
 
  out: