[Xen-devel] [PATCH] x86/PoD: tighten conditions for checking super page

["Jan Beulich" <JBeulich@xxxxxxxx>]

Since calling the function isn't cheap, try to avoid the call when we
know up front it won't help; see the code comment for details on those
conditions.

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

--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -522,7 +522,6 @@ p2m_pod_decrease_reservation(struct doma
     if ( unlikely(d->is_dying) )
         goto out_unlock;
 
-recount:
     pod = nonpod = ram = 0;
 
     /* Figure out if we need to steal some freed memory for our cache */
@@ -562,15 +561,20 @@ recount:
         goto out_entry_check;
     }
 
-    /* Try to grab entire superpages if possible.  Since the common case is 
for drivers
-     * to pass back singleton pages, see if we can take the whole page back 
and mark the
-     * rest PoD. */
-    if ( steal_for_cache
-         && p2m_pod_zero_check_superpage(p2m, gpfn & ~(SUPERPAGE_PAGES-1)))
-    {
-        /* Since order may be arbitrary, we may have taken more or less
-         * than we were actually asked to; so just re-count from scratch */
-        goto recount;
+    /*
+     * Try to grab entire superpages if possible.  Since the common case is for
+     * drivers to pass back singleton pages, see if we can take the whole page
+     * back and mark the rest PoD.
+     * No need to do this though if
+     * - order >= SUPERPAGE_ORDER (the loop below will take care of this)
+     * - not all of the pages were RAM (now knowing order < SUPERPAGE_ORDER)
+     */
+    if ( steal_for_cache && order < SUPERPAGE_ORDER && (ram >> order) &&
+         p2m_pod_zero_check_superpage(p2m, gpfn & ~(SUPERPAGE_PAGES - 1)) )
+    {
+        pod += ram;
+        nonpod -= ram;
+        ram = 0;
     }
 
     /* Process as long as:

Attachment: x86-PoD-emerg-avoid-check-super.patch
Description: Text document

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