mm: unmap page for direct mapped domain on decrease reservation

Direct mapped domain needs to retrieve the exact same underlying
physical page when the region is re-populated.

Currently, when the memory reservation for this domain is decreased, the
request is just ignored and the page stayed mapped in the P2M. However,
this make more difficult to spot issue when the domain has not yet mapped
foreign page but trying to access the region.

What we really care for direct mapped domain is to not give back the
page to the allocator. So we can re-enable to direct mapped when the guest
memory region is re-populated.

The rest of the process to remove a page can be safely done. This
also ensures us to stay close to the normal domain memory handling.

At the same time, drop the trailing whitespaces around the code
modified.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/memory.c

@@ -257,8 +257,17 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
 
     if ( test_and_clear_bit(_PGT_pinned, &page->u.inuse.type_info) )
         put_page_and_type(page);
-            
-    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
+
+    /*
+     * With the lack of an IOMMU on some platforms, domains with DMA-capable
+     * device must retrieve the same pfn when the hypercall populate_physmap
+     * is called.
+     *
+     * For this purpose (and to match populate_physmap() behavior), the page
+     * is kept allocated.
+     */
+    if ( !is_domain_direct_mapped(d) &&
+         test_and_clear_bit(_PGC_allocated, &page->count_info) )
         put_page(page);
 
     guest_physmap_remove_page(d, gmfn, mfn, 0);
@@ -309,13 +318,6 @@ static void decrease_reservation(struct memop_args *a)
              && p2m_pod_decrease_reservation(a->domain, gmfn, a->extent_order) )
             continue;
 
-        /* With the lack for iommu on some ARM platform, domain with DMA-capable
-         * device must retrieve the same pfn when the hypercall
-         * populate_physmap is called.
-         */
-        if ( is_domain_direct_mapped(a->domain) )
-            continue;
-
         for ( j = 0; j < (1 << a->extent_order); j++ )
             if ( !guest_remove_page(a->domain, gmfn + j) )
                 goto out;