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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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

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