[xen staging] xen/page_alloc: Simplify domain_adjust_tot_pages

[patchbot@xxxxxxx]

commit 3472783c266e52f630f29f0277b86c4dc24d9054
Author:     Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
AuthorDate: Tue Mar 4 11:10:00 2025 +0000
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Wed Mar 12 13:32:30 2025 +0100

    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@xxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 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 1bf070c8c5..5f9c9305ef 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:
--
generated by git-patchbot for /home/xen/git/xen.git#staging