pagewalk: improve vma handling

Current implementation of page table walker has a fundamental problem in
vma handling, which started when we tried to handle vma(VM_HUGETLB).
Because it's done in pgd loop, considering vma boundary makes code
complicated and bug-prone.

From the users viewpoint, some user checks some vma-related condition to
determine whether the user really does page walk over the vma.

In order to solve these, this patch moves vma check outside pgd loop and
introduce a new callback ->test_walk().

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 117 insertions, 89 deletions

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index b793ef149da2..d9cc3caae802 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -59,7 +59,7 @@ again:
                         continue;
 
                 split_huge_page_pmd_mm(walk->mm, addr, pmd);
-                if (pmd_none_or_trans_huge_or_clear_bad(pmd))
+                if (pmd_trans_unstable(pmd))
                         goto again;
                 err = walk_pte_range(pmd, addr, next, walk);
                 if (err)
@@ -95,6 +95,32 @@ static int walk_pud_range(pgd_t *pgd, unsigned long addr, unsigned long end,
         return err;
 }
 
+static int walk_pgd_range(unsigned long addr, unsigned long end,
+                          struct mm_walk *walk)
+{
+        pgd_t *pgd;
+        unsigned long next;
+        int err = 0;
+
+        pgd = pgd_offset(walk->mm, addr);
+        do {
+                next = pgd_addr_end(addr, end);
+                if (pgd_none_or_clear_bad(pgd)) {
+                        if (walk->pte_hole)
+                                err = walk->pte_hole(addr, next, walk);
+                        if (err)
+                                break;
+                        continue;
+                }
+                if (walk->pmd_entry || walk->pte_entry)
+                        err = walk_pud_range(pgd, addr, next, walk);
+                if (err)
+                        break;
+        } while (pgd++, addr = next, addr != end);
+
+        return err;
+}
+
 #ifdef CONFIG_HUGETLB_PAGE
 static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
                                        unsigned long end)
@@ -103,10 +129,10 @@ static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
         return boundary < end ? boundary : end;
 }
 
-static int walk_hugetlb_range(struct vm_area_struct *vma,
-                              unsigned long addr, unsigned long end,
+static int walk_hugetlb_range(unsigned long addr, unsigned long end,
                               struct mm_walk *walk)
 {
+        struct vm_area_struct *vma = walk->vma;
         struct hstate *h = hstate_vma(vma);
         unsigned long next;
         unsigned long hmask = huge_page_mask(h);
@@ -119,15 +145,14 @@ static int walk_hugetlb_range(struct vm_area_struct *vma,
                 if (pte && walk->hugetlb_entry)
                         err = walk->hugetlb_entry(pte, hmask, addr, next, walk);
                 if (err)
-                        return err;
+                        break;
         } while (addr = next, addr != end);
 
-        return 0;
+        return err;
 }
 
 #else /* CONFIG_HUGETLB_PAGE */
-static int walk_hugetlb_range(struct vm_area_struct *vma,
-                              unsigned long addr, unsigned long end,
+static int walk_hugetlb_range(unsigned long addr, unsigned long end,
                               struct mm_walk *walk)
 {
         return 0;
@@ -135,112 +160,115 @@ static int walk_hugetlb_range(struct vm_area_struct *vma,
 
 #endif /* CONFIG_HUGETLB_PAGE */
 
+/*
+ * Decide whether we really walk over the current vma on [@start, @end)
+ * or skip it via the returned value. Return 0 if we do walk over the
+ * current vma, and return 1 if we skip the vma. Negative values means
+ * error, where we abort the current walk.
+ *
+ * Default check (only VM_PFNMAP check for now) is used when the caller
+ * doesn't define test_walk() callback.
+ */
+static int walk_page_test(unsigned long start, unsigned long end,
+                        struct mm_walk *walk)
+{
+        struct vm_area_struct *vma = walk->vma;
 
+        if (walk->test_walk)
+                return walk->test_walk(start, end, walk);
+
+        /*
+         * Do not walk over vma(VM_PFNMAP), because we have no valid struct
+         * page backing a VM_PFNMAP range. See also commit a9ff785e4437.
+         */
+        if (vma->vm_flags & VM_PFNMAP)
+                return 1;
+        return 0;
+}
+
+static int __walk_page_range(unsigned long start, unsigned long end,
+                        struct mm_walk *walk)
+{
+        int err = 0;
+        struct vm_area_struct *vma = walk->vma;
+
+        if (vma && is_vm_hugetlb_page(vma)) {
+                if (walk->hugetlb_entry)
+                        err = walk_hugetlb_range(start, end, walk);
+        } else
+                err = walk_pgd_range(start, end, walk);
+
+        return err;
+}
 
 /**
- * walk_page_range - walk a memory map's page tables with a callback
- * @addr: starting address
- * @end: ending address
- * @walk: set of callbacks to invoke for each level of the tree
- *
- * Recursively walk the page table for the memory area in a VMA,
- * calling supplied callbacks. Callbacks are called in-order (first
- * PGD, first PUD, first PMD, first PTE, second PTE... second PMD,
- * etc.). If lower-level callbacks are omitted, walking depth is reduced.
+ * walk_page_range - walk page table with caller specific callbacks
  *
- * Each callback receives an entry pointer and the start and end of the
- * associated range, and a copy of the original mm_walk for access to
- * the ->private or ->mm fields.
+ * Recursively walk the page table tree of the process represented by @walk->mm
+ * within the virtual address range [@start, @end). During walking, we can do
+ * some caller-specific works for each entry, by setting up pmd_entry(),
+ * pte_entry(), and/or hugetlb_entry(). If you don't set up for some of these
+ * callbacks, the associated entries/pages are just ignored.
+ * The return values of these callbacks are commonly defined like below:
+ *  - 0  : succeeded to handle the current entry, and if you don't reach the
+ *         end address yet, continue to walk.
+ *  - >0 : succeeded to handle the current entry, and return to the caller
+ *         with caller specific value.
+ *  - <0 : failed to handle the current entry, and return to the caller
+ *         with error code.
  *
- * Usually no locks are taken, but splitting transparent huge page may
- * take page table lock. And the bottom level iterator will map PTE
- * directories from highmem if necessary.
+ * Before starting to walk page table, some callers want to check whether
+ * they really want to walk over the current vma, typically by checking
+ * its vm_flags. walk_page_test() and @walk->test_walk() are used for this
+ * purpose.
  *
- * If any callback returns a non-zero value, the walk is aborted and
- * the return value is propagated back to the caller. Otherwise 0 is returned.
+ * struct mm_walk keeps current values of some common data like vma and pmd,
+ * which are useful for the access from callbacks. If you want to pass some
+ * caller-specific data to callbacks, @walk->private should be helpful.
  *
- * walk->mm->mmap_sem must be held for at least read if walk->hugetlb_entry
- * is !NULL.
+ * Locking:
+ *   Callers of walk_page_range() and walk_page_vma() should hold
+ *   @walk->mm->mmap_sem, because these function traverse vma list and/or
+ *   access to vma's data.
  */
-int walk_page_range(unsigned long addr, unsigned long end,
+int walk_page_range(unsigned long start, unsigned long end,
                     struct mm_walk *walk)
 {
-        pgd_t *pgd;
-        unsigned long next;
         int err = 0;
+        unsigned long next;
+        struct vm_area_struct *vma;
 
-        if (addr >= end)
-                return err;
+        if (start >= end)
+                return -EINVAL;
 
         if (!walk->mm)
                 return -EINVAL;
 
         VM_BUG_ON_MM(!rwsem_is_locked(&walk->mm->mmap_sem), walk->mm);
 
-        pgd = pgd_offset(walk->mm, addr);
+        vma = find_vma(walk->mm, start);
         do {
-                struct vm_area_struct *vma = NULL;
+                if (!vma) { /* after the last vma */
+                        walk->vma = NULL;
+                        next = end;
+                } else if (start < vma->vm_start) { /* outside vma */
+                        walk->vma = NULL;
+                        next = min(end, vma->vm_start);
+                } else { /* inside vma */
+                        walk->vma = vma;
+                        next = min(end, vma->vm_end);
+                        vma = vma->vm_next;
 
-                next = pgd_addr_end(addr, end);
-
-                /*
-                 * This function was not intended to be vma based.
-                 * But there are vma special cases to be handled:
-                 * - hugetlb vma's
-                 * - VM_PFNMAP vma's
-                 */
-                vma = find_vma(walk->mm, addr);
-                if (vma) {
-                        /*
-                         * There are no page structures backing a VM_PFNMAP
-                         * range, so do not allow split_huge_page_pmd().
-                         */
-                        if ((vma->vm_start <= addr) &&
-                            (vma->vm_flags & VM_PFNMAP)) {
-                                if (walk->pte_hole)
-                                        err = walk->pte_hole(addr, next, walk);
-                                if (err)
-                                        break;
-                                pgd = pgd_offset(walk->mm, next);
+                        err = walk_page_test(start, next, walk);
+                        if (err > 0)
                                 continue;
-                        }
-                        /*
-                         * Handle hugetlb vma individually because pagetable
-                         * walk for the hugetlb page is dependent on the
-                         * architecture and we can't handled it in the same
-                         * manner as non-huge pages.
-                         */
-                        if (walk->hugetlb_entry && (vma->vm_start <= addr) &&
-                            is_vm_hugetlb_page(vma)) {
-                                if (vma->vm_end < next)
-                                        next = vma->vm_end;
-                                /*
-                                 * Hugepage is very tightly coupled with vma,
-                                 * so walk through hugetlb entries within a
-                                 * given vma.
-                                 */
-                                err = walk_hugetlb_range(vma, addr, next, walk);
-                                if (err)
-                                        break;
-                                pgd = pgd_offset(walk->mm, next);
-                                continue;
-                        }
-                }
-
-                if (pgd_none_or_clear_bad(pgd)) {
-                        if (walk->pte_hole)
-                                err = walk->pte_hole(addr, next, walk);
-                        if (err)
+                        if (err < 0)
                                 break;
-                        pgd++;
-                        continue;
                 }
-                if (walk->pmd_entry || walk->pte_entry)
-                        err = walk_pud_range(pgd, addr, next, walk);
+                if (walk->vma || walk->pte_hole)
+                        err = __walk_page_range(start, next, walk);
                 if (err)
                         break;
-                pgd++;
-        } while (addr = next, addr < end);
-
+        } while (start = next, start < end);
         return err;
 }