Re: [rfc][patch] mm: use a pte bit to flag normal pages

[Carsten Otte <cotte@de.ibm.com>]

Nick Piggin wrote:
> We initially wanted to do the whole vm_normal_page thing this way, with another
> pte bit, but we thought there were one or two archs with no spare bits. BTW. I
> also need this bit in order to implement my lockless get_user_pages, so I do hope
> to get it in. I'd like to know what architectures cannot spare a software bit in
> their pte_present ptes...
I've been playing with the original PAGE_SPECIAL patch a little bit, and
you can find the corresponding s390 definition below that you might want
to add to your patch queue.
It is a little unclear to me, how you'd like to proceed from here:
- with PTE_SPECIAL, do we still have VM_MIXEDMAP or similar flag to
distinguish our new type of mapping from VM_PFNMAP? Which vma flags are
we supposed to use for xip mappings?
- does VM_PFNMAP work as before, or do you intend to replace it?
- what about vm_normal_page? Do you intend to have one per arch? The one
proposed by this patch breaks Jared's pfn_valid() thing and VM_PFNMAP
for archs that don't have PAGE_SPECIAL as far as I can tell.

---
Index: linux-2.6/include/asm-s390/pgtable.h
===================================================================
--- linux-2.6.orig/include/asm-s390/pgtable.h
+++ linux-2.6/include/asm-s390/pgtable.h
@@ -228,6 +228,7 @@ extern unsigned long vmalloc_end;
 /* Software bits in the page table entry */
 #define _PAGE_SWT	0x001		/* SW pte type bit t */
 #define _PAGE_SWX	0x002		/* SW pte type bit x */
+#define _PAGE_SPECIAL	0x004		/* SW associated with special page */
 
 /* Six different types of pages. */
 #define _PAGE_TYPE_EMPTY	0x400
@@ -504,6 +505,12 @@ static inline int pte_file(pte_t pte)
 	return (pte_val(pte) & mask) == _PAGE_TYPE_FILE;
 }
 
+static inline int pte_special(pte_t pte)
+{
+	BUG_ON(!pte_present(pte));
+	return (pte_val(pte) & _PAGE_SPECIAL);
+}
+
 #define __HAVE_ARCH_PTE_SAME
 #define pte_same(a,b)  (pte_val(a) == pte_val(b))
 
@@ -654,6 +661,13 @@ static inline pte_t pte_mkyoung(pte_t pt
 	return pte;
 }
 
+static inline pte_t pte_mkspecial(pte_t pte)
+{
+	BUG_ON(!pte_present(pte));
+	pte_val(pte) |= _PAGE_SPECIAL;
+	return pte;
+}
+
 #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
 static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
 					    unsigned long addr, pte_t *ptep)


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