x86, mm: introduce _PAGE_DEVMAP

_PAGE_DEVMAP is a hardware-unused pte bit that will later be used in the
get_user_pages() path to identify pfns backed by the dynamic allocation
established by devm_memremap_pages.  Upon seeing that bit the gup path
will lookup and pin the allocation while the pages are in use.

Since the _PAGE_DEVMAP bit is > 32 it must be cast to u64 instead of a
pteval_t to allow pmd_flags() usage in the realmode boot code to build.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/pgtable_types.h

@@ -24,7 +24,9 @@
 #define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW1
 #define _PAGE_BIT_HIDDEN _PAGE_BIT_SOFTW3 /* hidden by kmemcheck */
 #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
-#define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */
+#define _PAGE_BIT_SOFTW4 58 /* available for programmer */
+#define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
+#define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */
 
 /* If _PAGE_BIT_PRESENT is clear, we use these: */
 /* - if the user mapped it with PROT_NONE; pte_present gives true */
@@ -83,8 +85,11 @@
 
 #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
 #define _PAGE_NX (_AT(pteval_t, 1) << _PAGE_BIT_NX)
+#define _PAGE_DEVMAP (_AT(u64, 1) << _PAGE_BIT_DEVMAP)
+#define __HAVE_ARCH_PTE_DEVMAP
 #else
 #define _PAGE_NX (_AT(pteval_t, 0))
+#define _PAGE_DEVMAP (_AT(pteval_t, 0))
 #endif
 
 #define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)