[Xen-devel] fix inconsistency between change_page_attr and phys_pud_init in x86-64

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

Other than native x86-64 code, the XEN guest *does* use 4k mappings for the 
contiguous kernel mapping of (physical) memory. Thus the code in 
change_page_attr needs to work like on i386 (where large pages are used only 
conditionally) rather than like native x86-64. Patch below/attached. Not doing 
so triggered the BUG_ON during load of intel-agp on machines with i915 chipset.

Jan

diff -Nprux .list 2.6.11-xen/x86_64/mm/pageattr.c 
nxen-guest/x86_64/mm/pageattr.c
--- 2.6.11-xen/arch/xen/x86_64/mm/pageattr.c    2005-07-01 09:14:36.606227616 
-0700
+++ nxen-guest/arch/xen/x86_64/mm/pageattr.c    2005-07-01 09:15:43.387075384 
-0700
@@ -166,16 +166,23 @@ __change_page_attr(unsigned long address
                BUG();
 
        /* on x86-64 the direct mapping set at boot is not using 4k pages */
-       BUG_ON(PageReserved(kpte_page));
-
-       switch (page_count(kpte_page)) {
-       case 1:
-               save_page(address, kpte_page);               
-               revert_page(address, ref_prot);
-               break;
-       case 0:
-               BUG(); /* memleak and failed 2M page regeneration */
-       }
+//     BUG_ON(PageReserved(kpte_page));
+       /*
+        * ..., but the XEN guest kernels (currently) do:
+        * If the pte was reserved, it means it was created at boot
+        * time (not via split_large_page) and in turn we must not
+        * replace it with a large page.
+        */
+       if (!PageReserved(kpte_page)) {
+               switch (page_count(kpte_page)) {
+               case 1:
+                       save_page(address, kpte_page);               
+                       revert_page(address, ref_prot);
+                       break;
+               case 0:
+                       BUG(); /* memleak and failed 2M page regeneration */
+               }
+       }
        return 0;
 }

Attachment: xenlinux-i915-agp.patch
Description: Binary data

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