[PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

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.

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

Not a functional change (although by its nature it might not seem
immediately obvious at first).

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
---
 xen/common/page_alloc.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 1bf070c8c5df..4306753f0bd2 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -490,13 +490,11 @@ 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
      */
@@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct domain *d, 
long pages)
         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 ( pages > 0 && 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:
-- 
2.48.1