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

Kirill A. Shutemov
2014-06-30T11:53:50Z

On Fri, Jun 20, 2014 at 04:11:28PM -0400, Naoya Horiguchi wrote:
> 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;

I guess it should be 'return err;', right?

> }
>
> #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;

Is there a reason why we shoul go for __walk_page_range() for these two
cases if walkj->pte_hole() is not defined?

Otherwise, looks okay.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

--
Kirill A. Shutemov