[Xen-devel] [PATCH 1 of 4] x86/mm: Make asserts on types and counts of shared pages more accurate

[Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>]

 xen/arch/x86/mm/mem_sharing.c |  3 ++-
 xen/arch/x86/mm/p2m.c         |  7 +++++--
 2 files changed, 7 insertions(+), 3 deletions(-)


Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

diff -r 658530e2f380 -r a70a87d7bf84 xen/arch/x86/mm/mem_sharing.c
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -201,7 +201,8 @@ static struct page_info* mem_sharing_loo
             /* Count has to be at least two, because we're called
              * with the mfn locked (1) and this is supposed to be 
              * a shared page (1). */
-            ASSERT((page->u.inuse.type_info & PGT_count_mask) >= 2); 
+            unsigned long type = read_atomic(&page->u.inuse.type_info);
+            ASSERT((type & PGT_shared_page) && ((type & PGT_count_mask) >= 2));
             ASSERT(get_gpfn_from_mfn(mfn) == SHARED_M2P_ENTRY); 
             return page;
         }
diff -r 658530e2f380 -r a70a87d7bf84 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -735,6 +735,7 @@ set_shared_p2m_entry(struct domain *d, u
     p2m_access_t a;
     p2m_type_t ot;
     mfn_t omfn;
+    unsigned long pg_type;
 
     if ( !paging_mode_translate(p2m->domain) )
         return 0;
@@ -745,8 +746,10 @@ set_shared_p2m_entry(struct domain *d, u
      * sharable first */
     ASSERT(p2m_is_shared(ot));
     ASSERT(mfn_valid(omfn));
-    if ( ((mfn_to_page(omfn)->u.inuse.type_info & PGT_type_mask) 
-                    != PGT_shared_page) )
+    /* Set the m2p entry to invalid only if there are no further type
+     * refs to this page as shared */
+    pg_type = read_atomic(&(mfn_to_page(omfn)->u.inuse.type_info));
+    if ( !((pg_type & PGT_shared_page) && (pg_type & PGT_count_mask)) )
         set_gpfn_from_mfn(mfn_x(omfn), INVALID_M2P_ENTRY);
 
     P2M_DEBUG("set shared %lx %lx\n", gfn, mfn_x(mfn));

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