mm: re-architect the VM_UNPAGED logic

This replaces the (in my opinion horrible) VM_UNMAPPED logic with very
explicit support for a "remapped page range" aka VM_PFNMAP.  It allows a
VM area to contain an arbitrary range of page table entries that the VM
never touches, and never considers to be normal pages.

Any user of "remap_pfn_range()" automatically gets this new
functionality, and doesn't even have to mark the pages reserved or
indeed mark them any other way.  It just works.  As a side effect, doing
mmap() on /dev/mem works for arbitrary ranges.

Sparc update from David in the next commit.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 100 insertions, 89 deletions

diff --git a/mm/memory.c b/mm/memory.c
index d1f46f4e4c8a..b57fbc636058 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -333,9 +333,9 @@ static inline void add_mm_rss(struct mm_struct *mm, int file_rss, int anon_rss)
 }
 
 /*
- * This function is called to print an error when a pte in a
- * !VM_UNPAGED region is found pointing to an invalid pfn (which
- * is an error.
+ * This function is called to print an error when a bad pte
+ * is found. For example, we might have a PFN-mapped pte in
+ * a region that doesn't allow it.
  *
  * The calling function must still handle the error.
  */
@@ -350,19 +350,56 @@ void print_bad_pte(struct vm_area_struct *vma, pte_t pte, unsigned long vaddr)
 }
 
 /*
- * page_is_anon applies strict checks for an anonymous page belonging to
- * this vma at this address.  It is used on VM_UNPAGED vmas, which are
- * usually populated with shared originals (which must not be counted),
- * but occasionally contain private COWed copies (when !VM_SHARED, or
- * perhaps via ptrace when VM_SHARED).  An mmap of /dev/mem might window
- * free pages, pages from other processes, or from other parts of this:
- * it's tricky, but try not to be deceived by foreign anonymous pages.
+ * This function gets the "struct page" associated with a pte.
+ *
+ * NOTE! Some mappings do not have "struct pages". A raw PFN mapping
+ * will have each page table entry just pointing to a raw 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".
+ *
+ * The way we recognize those 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
+ *
+ *      pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
+ *
+ * and if that isn't true, the page has been COW'ed (in which case it
+ * _does_ have a "struct page" associated with it even if it is in a
+ * VM_PFNMAP range).
  */
-static inline int page_is_anon(struct page *page,
-                        struct vm_area_struct *vma, unsigned long addr)
+struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_t pte)
 {
-        return page && PageAnon(page) && page_mapped(page) &&
-                page_address_in_vma(page, vma) == addr;
+        unsigned long pfn = pte_pfn(pte);
+
+        if (vma->vm_flags & VM_PFNMAP) {
+                unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
+                if (pfn == vma->vm_pgoff + off)
+                        return NULL;
+        }
+
+        /*
+         * 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.
+         *
+         * Remove this test eventually!
+         */
+        if (unlikely(!pfn_valid(pfn))) {
+                print_bad_pte(vma, pte, addr);
+                return NULL;
+        }
+
+        /*
+         * NOTE! We still have PageReserved() pages in the page 
+         * tables. 
+         *
+         * The PAGE_ZERO() pages and various VDSO mappings can
+         * cause them to exist.
+         */
+        return pfn_to_page(pfn);
 }
 
 /*
@@ -379,7 +416,6 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         unsigned long vm_flags = vma->vm_flags;
         pte_t pte = *src_pte;
         struct page *page;
-        unsigned long pfn;
 
         /* pte contains position in swap or file, so copy. */
         if (unlikely(!pte_present(pte))) {
@@ -397,22 +433,6 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                 goto out_set_pte;
         }
 
-        pfn = pte_pfn(pte);
-        page = pfn_valid(pfn)? pfn_to_page(pfn): NULL;
-
-        if (unlikely(vm_flags & VM_UNPAGED))
-                if (!page_is_anon(page, vma, addr))
-                        goto out_set_pte;
-
-        /*
-         * If the pte points outside of valid memory but
-         * the region is not VM_UNPAGED, we have a problem.
-         */
-        if (unlikely(!page)) {
-                print_bad_pte(vma, pte, addr);
-                goto out_set_pte; /* try to do something sane */
-        }
-
         /*
          * If it's a COW mapping, write protect it both
          * in the parent and the child
@@ -429,9 +449,13 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         if (vm_flags & VM_SHARED)
                 pte = pte_mkclean(pte);
         pte = pte_mkold(pte);
-        get_page(page);
-        page_dup_rmap(page);
-        rss[!!PageAnon(page)]++;
+
+        page = vm_normal_page(vma, addr, pte);
+        if (page) {
+                get_page(page);
+                page_dup_rmap(page);
+                rss[!!PageAnon(page)]++;
+        }
 
 out_set_pte:
         set_pte_at(dst_mm, addr, dst_pte, pte);
@@ -543,7 +567,7 @@ int copy_page_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
          * readonly mappings. The tradeoff is that copy_page_range is more
          * efficient than faulting.
          */
-        if (!(vma->vm_flags & (VM_HUGETLB|VM_NONLINEAR|VM_UNPAGED))) {
+        if (!(vma->vm_flags & (VM_HUGETLB|VM_NONLINEAR|VM_PFNMAP))) {
                 if (!vma->anon_vma)
                         return 0;
         }
@@ -584,19 +608,10 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
                 }
                 if (pte_present(ptent)) {
                         struct page *page;
-                        unsigned long pfn;
 
                         (*zap_work) -= PAGE_SIZE;
 
-                        pfn = pte_pfn(ptent);
-                        page = pfn_valid(pfn)? pfn_to_page(pfn): NULL;
-
-                        if (unlikely(vma->vm_flags & VM_UNPAGED)) {
-                                if (!page_is_anon(page, vma, addr))
-                                        page = NULL;
-                        } else if (unlikely(!page))
-                                print_bad_pte(vma, ptent, addr);
-
+                        page = vm_normal_page(vma, addr, ptent);
                         if (unlikely(details) && page) {
                                 /*
                                  * unmap_shared_mapping_pages() wants to
@@ -852,7 +867,7 @@ unsigned long zap_page_range(struct vm_area_struct *vma, unsigned long address,
 /*
  * Do a quick page-table lookup for a single page.
  */
-struct page *follow_page(struct mm_struct *mm, unsigned long address,
+struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
                         unsigned int flags)
 {
         pgd_t *pgd;
@@ -860,8 +875,8 @@ struct page *follow_page(struct mm_struct *mm, unsigned long address,
         pmd_t *pmd;
         pte_t *ptep, pte;
         spinlock_t *ptl;
-        unsigned long pfn;
         struct page *page;
+        struct mm_struct *mm = vma->vm_mm;
 
         page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
         if (!IS_ERR(page)) {
@@ -897,11 +912,10 @@ struct page *follow_page(struct mm_struct *mm, unsigned long address,
                 goto unlock;
         if ((flags & FOLL_WRITE) && !pte_write(pte))
                 goto unlock;
-        pfn = pte_pfn(pte);
-        if (!pfn_valid(pfn))
+        page = vm_normal_page(vma, address, pte);
+        if (unlikely(!page))
                 goto unlock;
 
-        page = pfn_to_page(pfn);
         if (flags & FOLL_GET)
                 get_page(page);
         if (flags & FOLL_TOUCH) {
@@ -974,8 +988,10 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                 return i ? : -EFAULT;
                         }
                         if (pages) {
-                                pages[i] = pte_page(*pte);
-                                get_page(pages[i]);
+                                struct page *page = vm_normal_page(vma, start, *pte);
+                                pages[i] = page;
+                                if (page)
+                                        get_page(page);
                         }
                         pte_unmap(pte);
                         if (vmas)
@@ -1010,7 +1026,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                 foll_flags |= FOLL_WRITE;
 
                         cond_resched();
-                        while (!(page = follow_page(mm, start, foll_flags))) {
+                        while (!(page = follow_page(vma, start, foll_flags))) {
                                 int ret;
                                 ret = __handle_mm_fault(mm, vma, start,
                                                 foll_flags & FOLL_WRITE);
@@ -1214,11 +1230,12 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
          *      in 2.6 the LRU scan won't even find its pages, so this
          *      flag means no more than count its pages in reserved_vm,
          *      and omit it from core dump, even when VM_IO turned off.
-         *   VM_UNPAGED tells the core MM not to "manage" these pages
-         *      (e.g. refcount, mapcount, try to swap them out): in
-         *      particular, zap_pte_range does not try to free them.
+         *   VM_PFNMAP tells the core MM that the base pages are just
+         *      raw PFN mappings, and do not have a "struct page" associated
+         *      with them.
          */
-        vma->vm_flags |= VM_IO | VM_RESERVED | VM_UNPAGED;
+        vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;
+        vma->vm_pgoff = pfn;
 
         BUG_ON(addr >= end);
         pfn -= addr >> PAGE_SHIFT;
@@ -1273,6 +1290,26 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
         return pte;
 }
 
+static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va)
+{
+        /*
+         * If the source page was a PFN mapping, we don't have
+         * a "struct page" for it. We do a best-effort copy by
+         * just copying from the original user address. If that
+         * fails, we just zero-fill it. Live with it.
+         */
+        if (unlikely(!src)) {
+                void *kaddr = kmap_atomic(dst, KM_USER0);
+                unsigned long left = __copy_from_user_inatomic(kaddr, (void __user *)va, PAGE_SIZE);
+                if (left)
+                        memset(kaddr, 0, PAGE_SIZE);
+                kunmap_atomic(kaddr, KM_USER0);
+                return;
+                
+        }
+        copy_user_highpage(dst, src, va);
+}
+
 /*
  * This routine handles present pages, when users try to write
  * to a shared page. It is done by copying the page to a new address
@@ -1296,28 +1333,13 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
                 spinlock_t *ptl, pte_t orig_pte)
 {
         struct page *old_page, *src_page, *new_page;
-        unsigned long pfn = pte_pfn(orig_pte);
         pte_t entry;
         int ret = VM_FAULT_MINOR;
 
-        if (unlikely(!pfn_valid(pfn))) {
-                /*
-                 * Page table corrupted: show pte and kill process.
-                 * Or it's an attempt to COW an out-of-map VM_UNPAGED
-                 * entry, which copy_user_highpage does not support.
-                 */
-                print_bad_pte(vma, orig_pte, address);
-                ret = VM_FAULT_OOM;
-                goto unlock;
-        }
-        old_page = pfn_to_page(pfn);
+        old_page = vm_normal_page(vma, address, orig_pte);
         src_page = old_page;
-
-        if (unlikely(vma->vm_flags & VM_UNPAGED))
-                if (!page_is_anon(old_page, vma, address)) {
-                        old_page = NULL;
-                        goto gotten;
-                }
+        if (!old_page)
+                goto gotten;
 
         if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
                 int reuse = can_share_swap_page(old_page);
@@ -1351,7 +1373,7 @@ gotten:
                 new_page = alloc_page_vma(GFP_HIGHUSER, vma, address);
                 if (!new_page)
                         goto oom;
-                copy_user_highpage(new_page, src_page, address);
+                cow_user_page(new_page, src_page, address);
         }
 
         /*
@@ -1812,16 +1834,7 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
         spinlock_t *ptl;
         pte_t entry;
 
-        /*
-         * A VM_UNPAGED vma will normally be filled with present ptes
-         * by remap_pfn_range, and never arrive here; but it might have
-         * holes, or if !VM_DONTEXPAND, mremap might have expanded it.
-         * It's weird enough handling anon pages in unpaged vmas, we do
-         * not want to worry about ZERO_PAGEs too (it may or may not
-         * matter if their counts wrap): just give them anon pages.
-         */
-
-        if (write_access || (vma->vm_flags & VM_UNPAGED)) {
+        if (write_access) {
                 /* Allocate our own private page. */
                 pte_unmap(page_table);
 
@@ -1896,8 +1909,6 @@ static int do_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
         int anon = 0;
 
         pte_unmap(page_table);
-        BUG_ON(vma->vm_flags & VM_UNPAGED);
-
         if (vma->vm_file) {
                 mapping = vma->vm_file->f_mapping;
                 sequence = mapping->truncate_count;
@@ -1930,7 +1941,7 @@ retry:
                 page = alloc_page_vma(GFP_HIGHUSER, vma, address);
                 if (!page)
                         goto oom;
-                copy_user_highpage(page, new_page, address);
+                cow_user_page(page, new_page, address);
                 page_cache_release(new_page);
                 new_page = page;
                 anon = 1;