1 files changed, 55 insertions, 44 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 0da414c383e7..c5e88bcd8ec3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -371,33 +371,37 @@ static inline int is_cow_mapping(unsigned int flags)
 }
 
 /*
- * This function gets the "struct page" associated with a pte or returns
- * NULL if no "struct page" is associated with the pte.
+ * vm_normal_page -- This function gets the "struct page" associated with a pte.
  *
- * A raw VM_PFNMAP mapping (ie. one that is not COWed) may not have any "struct
- * page" backing, and even if they do, they are not refcounted. COWed pages of
- * a VM_PFNMAP do always have a struct page, and they are normally refcounted
- * (they are _normal_ pages).
+ * "Special" mappings do not wish to be associated with a "struct page" (either
+ * it doesn't exist, or it exists but they don't want to touch it). In this
+ * case, NULL is returned here. "Normal" mappings do have a struct page.
  *
- * So a raw PFNMAP mapping will have each page table entry just pointing
- * to a page frame number, and as far as the VM layer is concerned, those do
- * not have pages associated with them - even if the PFN might point to memory
- * that otherwise is perfectly fine and has a "struct page".
+ * There are 2 broad cases. Firstly, an architecture may define a pte_special()
+ * pte bit, in which case this function is trivial. Secondly, an architecture
+ * may not have a spare pte bit, which requires a more complicated scheme,
+ * described below.
+ *
+ * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
+ * special mapping (even if there are underlying and valid "struct pages").
+ * COWed pages of a VM_PFNMAP are always normal.
  *
  * The way we recognize COWed pages within VM_PFNMAP mappings is through the
  * rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit
- * set, and the vm_pgoff will point to the first PFN mapped: thus every
- * page that is a raw mapping will always honor the rule
+ * set, and the vm_pgoff will point to the first PFN mapped: thus every special
+ * mapping will always honor the rule
  *
  *      pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
  *
- * A call to vm_normal_page() will return NULL for such a page.
+ * And for normal mappings this is false.
+ *
+ * This restricts such mappings to be a linear translation from virtual address
+ * to pfn. To get around this restriction, we allow arbitrary mappings so long
+ * as the vma is not a COW mapping; in that case, we know that all ptes are
+ * special (because none can have been COWed).
  *
- * If the page doesn't follow the "remap_pfn_range()" rule in a VM_PFNMAP
- * then the page has been COW'ed.  A COW'ed page _does_ have a "struct page"
- * associated with it even if it is in a VM_PFNMAP range.  Calling
- * vm_normal_page() on such a page will therefore return the "struct page".
  *
+ * In order to support COW of arbitrary special mappings, we have VM_MIXEDMAP.
  *
  * VM_MIXEDMAP mappings can likewise contain memory with or without "struct
  * page" backing, however the difference is that _all_ pages with a struct
@@ -407,16 +411,29 @@ static inline int is_cow_mapping(unsigned int flags)
  * advantage is that we don't have to follow the strict linearity rule of
  * PFNMAP mappings in order to support COWable mappings.
  *
- * A call to vm_normal_page() with a VM_MIXEDMAP mapping will return the
- * associated "struct page" or NULL for memory not backed by a "struct page".
- *
- *
- * All other mappings should have a valid struct page, which will be
- * returned by a call to vm_normal_page().
  */
-struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_t pte)
+#ifdef __HAVE_ARCH_PTE_SPECIAL
+# define HAVE_PTE_SPECIAL 1
+#else
+# define HAVE_PTE_SPECIAL 0
+#endif
+struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+                                pte_t pte)
 {
-        unsigned long pfn = pte_pfn(pte);
+        unsigned long pfn;
+
+        if (HAVE_PTE_SPECIAL) {
+                if (likely(!pte_special(pte))) {
+                        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+                        return pte_page(pte);
+                }
+                VM_BUG_ON(!(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)));
+                return NULL;
+        }
+
+        /* !HAVE_PTE_SPECIAL case follows: */
+
+        pfn = pte_pfn(pte);
 
         if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
                 if (vma->vm_flags & VM_MIXEDMAP) {
@@ -424,7 +441,8 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_
                                 return NULL;
                         goto out;
                 } else {
-                        unsigned long off = (addr-vma->vm_start) >> PAGE_SHIFT;
+                        unsigned long off;
+                        off = (addr - vma->vm_start) >> PAGE_SHIFT;
                         if (pfn == vma->vm_pgoff + off)
                                 return NULL;
                         if (!is_cow_mapping(vma->vm_flags))
@@ -432,25 +450,12 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_
                 }
         }
 
-#ifdef CONFIG_DEBUG_VM
-        /*
-         * Add some anal sanity checks for now. Eventually,
-         * we should just do "return pfn_to_page(pfn)", but
-         * in the meantime we check that we get a valid pfn,
-         * and that the resulting page looks ok.
-         */
-        if (unlikely(!pfn_valid(pfn))) {
-                print_bad_pte(vma, pte, addr);
-                return NULL;
-        }
-#endif
+        VM_BUG_ON(!pfn_valid(pfn));
 
         /*
-         * NOTE! We still have PageReserved() pages in the page 
-         * tables. 
+         * NOTE! We still have PageReserved() pages in the page tables.
          *
-         * The PAGE_ZERO() pages and various VDSO mappings can
-         * cause them to exist.
+         * eg. VDSO mappings can cause them to exist.
          */
 out:
         return pfn_to_page(pfn);
@@ -1263,6 +1268,12 @@ int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
         pte_t *pte, entry;
         spinlock_t *ptl;
 
+        /*
+         * Technically, architectures with pte_special can avoid all these
+         * restrictions (same for remap_pfn_range).  However we would like
+         * consistency in testing and feature parity among all, so we should
+         * try to keep these invariants in place for everybody.
+         */
         BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
         BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
                                                 (VM_PFNMAP|VM_MIXEDMAP));
@@ -1278,7 +1289,7 @@ int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
                 goto out_unlock;
 
         /* Ok, finally just insert the thing.. */
-        entry = pfn_pte(pfn, vma->vm_page_prot);
+        entry = pte_mkspecial(pfn_pte(pfn, vma->vm_page_prot));
         set_pte_at(mm, addr, pte, entry);
         update_mmu_cache(vma, addr, entry);
 
@@ -1309,7 +1320,7 @@ static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
         arch_enter_lazy_mmu_mode();
         do {
                 BUG_ON(!pte_none(*pte));
-                set_pte_at(mm, addr, pte, pfn_pte(pfn, prot));
+                set_pte_at(mm, addr, pte, pte_mkspecial(pfn_pte(pfn, prot)));
                 pfn++;
         } while (pte++, addr += PAGE_SIZE, addr != end);
         arch_leave_lazy_mmu_mode();