xen/page_alloc: Simplify domain_adjust_tot_pages

The logic has too many levels of indirection and it's very hard to
understand it its current form. Split it between the corner case where
the adjustment is bigger than the current claim and the rest to avoid 5
auxiliary variables.

Add a functional change to prevent negative adjustments from
re-increasing the claim. This has the nice side effect of avoiding
taking the heap lock here on every free.

While at it, fix incorrect field name in nearby comment.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 1bf070c8c5df0f2427de794ab219060a90554430..5f9c9305ef3742ea7a6c8d83b636ad3c498b7acc 100644 (file)
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -490,31 +490,30 @@ static long outstanding_claims; /* total outstanding claims by all domains */
 
 unsigned long domain_adjust_tot_pages(struct domain *d, long pages)
 {
-    long dom_before, dom_after, dom_claimed, sys_before, sys_after;
-
     ASSERT(rspin_is_locked(&d->page_alloc_lock));
     d->tot_pages += pages;
 
     /*
-     * can test d->claimed_pages race-free because it can only change
+     * can test d->outstanding_pages race-free because it can only change
      * if d->page_alloc_lock and heap_lock are both held, see also
      * domain_set_outstanding_pages below
      */
-    if ( !d->outstanding_pages )
+    if ( !d->outstanding_pages || pages <= 0 )
         goto out;
 
     spin_lock(&heap_lock);
-    /* adjust domain outstanding pages; may not go negative */
-    dom_before = d->outstanding_pages;
-    dom_after = dom_before - pages;
-    BUG_ON(dom_before < 0);
-    dom_claimed = dom_after < 0 ? 0 : dom_after;
-    d->outstanding_pages = dom_claimed;
-    /* flag accounting bug if system outstanding_claims would go negative */
-    sys_before = outstanding_claims;
-    sys_after = sys_before - (dom_before - dom_claimed);
-    BUG_ON(sys_after < 0);
-    outstanding_claims = sys_after;
+    BUG_ON(outstanding_claims < d->outstanding_pages);
+    if ( d->outstanding_pages < pages )
+    {
+        /* `pages` exceeds the domain's outstanding count. Zero it out. */
+        outstanding_claims -= d->outstanding_pages;
+        d->outstanding_pages = 0;
+    }
+    else
+    {
+        outstanding_claims -= pages;
+        d->outstanding_pages -= pages;
+    }
     spin_unlock(&heap_lock);
 
 out: