[PATCH v2] xen/page_alloc: Simplify domain_adjust_tot_pages

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

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>
---
v1->v2:
  * Functional change by preventing negative adjustments from increasing
    the claim. Exit early on pages <= 0.
  * Reflect said functional change in commit message.
---
 xen/common/page_alloc.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 1bf070c8c5df..5f9c9305ef37 100644
--- 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:
-- 
2.48.1