mm.h: add helper function to test-and-clear _PGC_allocated

The _PGC_allocated flag is set on a page when it is assigned to a domain
along with an initial reference count of at least 1. To clear this
'allocation' reference it is necessary to test-and-clear _PGC_allocated and
then only drop the reference if the test-and-clear succeeds. This is open-
coded in many places. It is also unsafe to test-and-clear _PGC_allocated
unless the caller holds an additional reference.

This patch adds a helper function, put_page_alloc_ref(), to replace all the
open-coded test-and-clear/put_page occurrences. That helper function
incorporates a check that an additional page reference is held and will
BUG() if it is not.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/arm/domain.c

@@ -926,9 +926,7 @@ static int relinquish_memory(struct domain *d, struct page_list_head *list)
              */
             continue;
 
-        if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-            put_page(page);
-
+        put_page_alloc_ref(page);
         put_page(page);
 
         if ( hypercall_preempt_check() )

xen/arch/x86/domain.c

@@ -1939,8 +1939,7 @@ static int relinquish_memory(
             BUG();
         }
 
-        if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-            put_page(page);
+        put_page_alloc_ref(page);
 
         /*
          * Forcibly invalidate top-most, still valid page tables at this point

xen/arch/x86/hvm/ioreq.c

@@ -398,8 +398,7 @@ static int hvm_alloc_ioreq_mfn(struct hvm_ioreq_server *s, bool buf)
     return 0;
 
  fail:
-    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-        put_page(page);
+    put_page_alloc_ref(page);
     put_page_and_type(page);
 
     return -ENOMEM;
@@ -418,13 +417,7 @@ static void hvm_free_ioreq_mfn(struct hvm_ioreq_server *s, bool buf)
     unmap_domain_page_global(iorp->va);
     iorp->va = NULL;
 
-    /*
-     * Check whether we need to clear the allocation reference before
-     * dropping the explicit references taken by get_page_and_type().
-     */
-    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-        put_page(page);
-
+    put_page_alloc_ref(page);
     put_page_and_type(page);
 }
 

xen/arch/x86/mm.c

@@ -498,8 +498,7 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
 
 void free_shared_domheap_page(struct page_info *page)
 {
-    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-        put_page(page);
+    put_page_alloc_ref(page);
     if ( !test_and_clear_bit(_PGC_xen_heap, &page->count_info) )
         ASSERT_UNREACHABLE();
     page->u.inuse.type_info = 0;

xen/arch/x86/mm/mem_sharing.c

@@ -1000,8 +1000,7 @@ static int share_pages(struct domain *sd, gfn_t sgfn, shr_handle_t sh,
     mem_sharing_page_unlock(firstpg);
 
     /* Free the client page */
-    if(test_and_clear_bit(_PGC_allocated, &cpage->count_info))
-        put_page(cpage);
+    put_page_alloc_ref(cpage);
     put_page(cpage);
 
     /* We managed to free a domain page. */
@@ -1082,8 +1081,7 @@ int mem_sharing_add_to_physmap(struct domain *sd, unsigned long sgfn, shr_handle
                     ret = -EOVERFLOW;
                     goto err_unlock;
                 }
-                if ( test_and_clear_bit(_PGC_allocated, &cpage->count_info) )
-                    put_page(cpage);
+                put_page_alloc_ref(cpage);
                 put_page(cpage);
             }
         }
@@ -1177,8 +1175,7 @@ int __mem_sharing_unshare_page(struct domain *d,
                 domain_crash(d);
                 return -EOVERFLOW;
             }
-            if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-                put_page(page);
+            put_page_alloc_ref(page);
             put_page(page);
         }
         put_gfn(d, gfn);

xen/arch/x86/mm/p2m-pod.c

@@ -274,9 +274,7 @@ p2m_pod_set_cache_target(struct p2m_domain *p2m, unsigned long pod_target, int p
             if ( test_and_clear_bit(_PGT_pinned, &(page+i)->u.inuse.type_info) )
                 put_page_and_type(page + i);
 
-            if ( test_and_clear_bit(_PGC_allocated, &(page+i)->count_info) )
-                put_page(page + i);
-
+            put_page_alloc_ref(page + i);
             put_page(page + i);
 
             if ( preemptible && pod_target != p2m->pod.count &&

xen/arch/x86/mm/p2m.c

@@ -1609,8 +1609,7 @@ int p2m_mem_paging_evict(struct domain *d, unsigned long gfn_l)
         goto out_put;
 
     /* Decrement guest domain's ref count of the page */
-    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-        put_page(page);
+    put_page_alloc_ref(page);
 
     /* Remove mapping from p2m table */
     ret = p2m_set_entry(p2m, gfn, INVALID_MFN, PAGE_ORDER_4K,

xen/common/grant_table.c

@@ -1707,8 +1707,7 @@ gnttab_unpopulate_status_frames(struct domain *d, struct grant_table *gt)
         }
 
         BUG_ON(page_get_owner(pg) != d);
-        if ( test_and_clear_bit(_PGC_allocated, &pg->count_info) )
-            put_page(pg);
+        put_page_alloc_ref(pg);
 
         if ( pg->count_info & ~PGC_xen_heap )
         {

xen/common/memory.c

@@ -388,9 +388,8 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
      * For this purpose (and to match populate_physmap() behavior), the page
      * is kept allocated.
      */
-    if ( !rc && !is_domain_direct_mapped(d) &&
-         test_and_clear_bit(_PGC_allocated, &page->count_info) )
-        put_page(page);
+    if ( !rc && !is_domain_direct_mapped(d) )
+        put_page_alloc_ref(page);
 
     put_page(page);
 

xen/common/xenoprof.c

@@ -173,8 +173,7 @@ unshare_xenoprof_page_with_guest(struct xenoprof *x)
         struct page_info *page = mfn_to_page(mfn_add(mfn, i));
 
         BUG_ON(page_get_owner(page) != current->domain);
-        if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-            put_page(page);
+        put_page_alloc_ref(page);
     }
 }
 

xen/include/xen/mm.h

@@ -658,4 +658,18 @@ static inline void share_xen_page_with_privileged_guests(
     share_xen_page_with_guest(page, dom_xen, flags);
 }
 
+static inline void put_page_alloc_ref(struct page_info *page)
+{
+    /*
+     * Whenever a page is assigned to a domain then the _PGC_allocated bit
+     * is set and the reference count is set to at least 1. This function
+     * clears that 'allocation reference' but it is unsafe to do so without
+     * the caller holding an additional reference. I.e. the allocation
+     * reference must never be the last reference held.
+     */
+    BUG_ON((page->count_info & PGC_count_mask) <= 1);
+    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
+        put_page(page);
+}
+
 #endif /* __XEN_MM_H__ */