[RFC,for-next,16/18] x86/mem_sharing: check page type count earlier

Commit Message

Tamas K Lengyel Sept. 25, 2019, 3:48 p.m. UTC

---
 xen/arch/x86/mm/mem_sharing.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

Patch

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index d35b7eb138..f54969bcad 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -649,19 +649,18 @@  static int page_make_sharable(struct domain *d,
         return -EBUSY;
     }
 
-    /* Change page type and count atomically */
-    if ( !get_page_and_type(page, d, PGT_shared_page) )
+    /* Check if page is already typed and bail early if it is */
+    if ( (page->u.inuse.type_info & PGT_count_mask) != 1 )
     {
         spin_unlock(&d->page_alloc_lock);
-        return -EINVAL;
+        return -EEXIST;
     }
 
-    /* Check it wasn't already sharable and undo if it was */
-    if ( (page->u.inuse.type_info & PGT_count_mask) != 1 )
+    /* Change page type and count atomically */
+    if ( !get_page_and_type(page, d, PGT_shared_page) )
     {
         spin_unlock(&d->page_alloc_lock);
-        put_page_and_type(page);
-        return -EEXIST;
+        return -EINVAL;
     }
 
     /*