Date:	Mon, 30 Jun 2014 10:28:37 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v3 02/13] pagewalk: improve vma handling

On Mon, Jun 30, 2014 at 02:53:11PM +0300, Kirill A. Shutemov wrote:
> 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@...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?

Oh, I see, we can omit it.
I'll do it.

> 
> Otherwise, looks okay.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Thank you for your reviewing.

Naoya
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