Xen devel - Apr 2012 - [PATCH] x86/mem sharing: Take care of domain reference for shared pages

xen/arch/x86/mm/mem_sharing.c |  8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)


Making a page sharable removes it from the previous owner''s list.
Making it
private adds it. These actions are similar to freeing or allocating a page.
Except that they were not minding the domain reference that is taken/dropped
when the first/last page is allocated/freed.

Without fixing this, a domain might remain zombie when destroyed if all its
pages are shared.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

diff -r 48763a117b2a -r 6a3e651ee3b6 xen/arch/x86/mm/mem_sharing.c
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -401,6 +401,7 @@ static int page_make_sharable(struct dom
                        struct page_info *page, 
                        int expected_refcnt)
 {
+    int drop_dom_ref;
     spin_lock(&d->page_alloc_lock);
 
     /* Change page type and count atomically */
@@ -430,8 +431,12 @@ static int page_make_sharable(struct dom
 
     page_set_owner(page, dom_cow);
     d->tot_pages--;
+    drop_dom_ref = (d->tot_pages == 0);
     page_list_del(page, &d->page_list);
     spin_unlock(&d->page_alloc_lock);
+
+    if ( drop_dom_ref )
+        put_domain(d);
     return 0;
 }
 
@@ -466,7 +471,8 @@ static int page_make_private(struct doma
     ASSERT(page_get_owner(page) == dom_cow);
     page_set_owner(page, d);
 
-    d->tot_pages++;
+    if ( d->tot_pages++ == 0 )
+        get_domain(d);
     page_list_add_tail(page, &d->page_list);
     spin_unlock(&d->page_alloc_lock);

At 11:48 -0400 on 04 Apr (1333540112), Andres Lagar-Cavilla
wrote:
> Making a page sharable removes it from the previous owner''s list.
Making it
> private adds it. These actions are similar to freeing or allocating a page.
> Except that they were not minding the domain reference that is
taken/dropped
> when the first/last page is allocated/freed.
> 
> Without fixing this, a domain might remain zombie when destroyed if all its
> pages are shared.
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Applied, thanks.

Tim.