[PATCH] x86/mem-paging: further adjustments to p2m_mem_paging_prep()'s error handling

[Jan Beulich <jbeulich@xxxxxxxx>]

Address late comments on ecb913be4aaa ("x86/mem-paging: correct
p2m_mem_paging_prep()'s error handling"):
- insert a gprintk() ahead of domain_crash(),
- add a comment.

Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/mem_paging.c
+++ b/xen/arch/x86/mm/mem_paging.c
@@ -385,6 +385,9 @@ static int prepare(struct domain *d, gfn
              * The domain can't possibly know about this page yet, so failure
              * here is a clear indication of something fishy going on.
              */
+            gprintk(XENLOG_ERR,
+                    "%pd: fresh page for GFN %"PRI_gfn" in unexpected state\n",
+                    d, gfn_x(gfn));
             domain_crash(d);
             page = NULL;
             goto out;
@@ -423,6 +426,10 @@ static int prepare(struct domain *d, gfn
 
     if ( page )
     {
+        /*
+         * Free the page on error.  Drop our temporary reference in all
+         * cases.
+         */
         if ( ret )
             put_page_alloc_ref(page);
         put_page(page);