[PATCH v3 02/13] pagewalk: improve vma handling

Naoya Horiguchi
2014-06-20T20:12:07Z

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().

ChangeLog v3:
- drop walk->skip control

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
include/linux/mm.h | 15 +++-
mm/pagewalk.c | 198 ++++++++++++++++++++++++++++++-----------------------
2 files changed, 126 insertions(+), 87 deletions(-)

diff --git v3.16-rc1.orig/include/linux/mm.h v3.16-rc1/include/linux/mm.h
index c5cb6394e6cb..489a63a06a4a 100644
--- v3.16-rc1.orig/include/linux/mm.h
+++ v3.16-rc1/include/linux/mm.h
@@ -1107,10 +1107,16 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
* @pte_entry: if set, called for each non-empty PTE (4th-level) entry
* @pte_hole: if set, called for each hole at all levels
* @hugetlb_entry: if set, called for each hugetlb entry
- * *Caution*: The caller must hold mmap_sem() if @hugetlb_entry
- * is used.
+ * @test_walk: caller specific callback function to determine whether
+ * we walk over the current vma or not. A positive returned
+ * value means "do page table walk over the current vma,"
+ * and a negative one means "abort current page table walk
+ * right now." 0 means "skip the current vma."
+ * @mm: mm_struct representing the target process of page table walk
+ * @vma: vma currently walked (NULL if walking outside vmas)
+ * @private: private data for callbacks' usage
*
- * (see walk_page_range for more details)
+ * (see the comment on walk_page_range() for more details)
*/
struct mm_walk {
int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
@@ -1122,7 +1128,10 @@ struct mm_walk {
int (*hugetlb_entry)(pte_t *pte, unsigned long hmask,
unsigned long addr, unsigned long next,
struct mm_walk *walk);
+ int (*test_walk)(unsigned long addr, unsigned long next,
+ struct mm_walk *walk);
struct mm_struct *mm;
+ struct vm_area_struct *vma;
void *private;
};

diff --git v3.16-rc1.orig/mm/pagewalk.c v3.16-rc1/mm/pagewalk.c
index 335690650b12..86d811202374 100644
--- v3.16-rc1.orig/mm/pagewalk.c
+++ v3.16-rc1/mm/pagewalk.c
@@ -59,7 +59,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
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;
}

#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;
@@ -135,109 +160,114 @@ 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;

- if (addr >= end)
- return err;
+ if (start >= end)
+ return -EINVAL;

if (!walk->mm)
return -EINVAL;

VM_BUG_ON(!rwsem_is_locked(&walk->mm->mmap_sem));

- pgd = pgd_offset(walk->mm, addr);
do {
- struct vm_area_struct *vma = NULL;
+ struct vm_area_struct *vma;

- next = pgd_addr_end(addr, end);
+ vma = find_vma(walk->mm, start);
+ if (!vma) { /* after the last vma */
+ walk->vma = NULL;
+ next = end;
+ } else if (start < vma->vm_start) { /* outside the found vma */
+ walk->vma = NULL;
+ next = vma->vm_start;
+ } else { /* inside the found vma */
+ walk->vma = vma;
+ next = min(end, vma->vm_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)) {
- next = vma->vm_end;
- pgd = pgd_offset(walk->mm, next);
- 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);
+ err = walk_page_test(start, next, walk);
+ if (err > 0)
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);
+ err = __walk_page_range(start, next, walk);
if (err)
break;
- pgd++;
- } while (addr = next, addr < end);
-
+ } while (start = next, start < end);
return err;
}
--
1.9.3