Re: [PATCH 2/2] [BUGFIX] pagemap: fix pfn calculation for hugepage

[KAMEZAWA Hiroyuki]

On Fri, 19 Mar 2010 15:26:36 +0900
Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:

> When we look into pagemap using page-types with option -p, the value
> of pfn for hugepages looks wrong (see below.)
> This is because pte was evaluated only once for one vma
> although it should be updated for each hugepage. This patch fixes it.
> 
> $ page-types -p 3277 -Nl -b huge
> voffset   offset  len     flags
> 7f21e8a00 11e400  1       ___U___________H_G________________
> 7f21e8a01 11e401  1ff     ________________TG________________
> 7f21e8c00 11e400  1       ___U___________H_G________________
> 7f21e8c01 11e401  1ff     ________________TG________________
>              ^^^
>              should not be the same
> 
> With this patch applied:
> 
> $ page-types -p 3386 -Nl -b huge
> voffset   offset   len    flags
> 7fec7a600 112c00   1      ___UD__________H_G________________
> 7fec7a601 112c01   1ff    ________________TG________________
> 7fec7a800 113200   1      ___UD__________H_G________________
> 7fec7a801 113201   1ff    ________________TG________________
>              ^^^
>              OK
> 
Hmm. Is this bug ? To me, it's just shown in hugepage's pagesize, by design.

_And_, Doesn't this patch change behavior of walk_pagemap_range() implicitly ?
No influence to other users ? (as memcontrol.c. in mmotm. Ask Nishimura-san ;)


some nitpicks.


> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> ---
>  fs/proc/task_mmu.c |   37 +++++++++++++++++++------------------
>  include/linux/mm.h |    4 ++--
>  mm/pagewalk.c      |   14 ++++----------
>  3 files changed, 25 insertions(+), 30 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 2a3ef17..cc14479 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -662,31 +662,32 @@ static u64 huge_pte_to_pagemap_entry(pte_t pte, int offset)
>  	return pme;
>  }
>  
> -static int pagemap_hugetlb_range(pte_t *pte, unsigned long addr,
> +/* This function walks only within @vma */
> +static int pagemap_hugetlb_range(struct vm_area_struct *vma, unsigned long addr,
>  				 unsigned long end, struct mm_walk *walk)
>  {
> -	struct vm_area_struct *vma;
> +	struct mm_struct *mm = walk->mm;
>  	struct pagemapread *pm = walk->private;
>  	struct hstate *hs = NULL;
>  	int err = 0;
> -
> -	vma = find_vma(walk->mm, addr);
> -	if (vma)
> -		hs = hstate_vma(vma);
> +	pte_t *pte = NULL;
> +
> +	BUG_ON(!mm);
> +	BUG_ON(!vma || !is_vm_hugetlb_page(vma));
> +	BUG_ON(addr < vma->vm_start || addr >= vma->vm_end);

This is my personal opinion, may not be popular.

When you add BUG_ON(), please confirm "you have real concern about this."
After reading this, code reader will take care of avoiding calling this
function with above condition. yes.

But, this function itself is only for pagemap_read() and it seems no
other one will call this function externally in future.
Above 3 BUG_ON will never happen because of simple logic around this.
Then, it seems unnecessary noise to me.

If your changes in walk_page_range() causes concerns to add above BUG_ON()s,
please avoid such changes. 

Bye.
-Kame

> +	hs = hstate_vma(vma);
> +	BUG_ON(!hs);
> +	pte = huge_pte_offset(mm, addr);
> +	if (!pte)
> +		return err;
>  	for (; addr != end; addr += PAGE_SIZE) {
>  		u64 pfn = PM_NOT_PRESENT;
> -
> -		if (vma && (addr >= vma->vm_end)) {
> -			vma = find_vma(walk->mm, addr);
> -			if (vma)
> -				hs = hstate_vma(vma);
> -		}
> -
> -		if (vma && (vma->vm_start <= addr) && is_vm_hugetlb_page(vma)) {
> -			/* calculate pfn of the "raw" page in the hugepage. */
> -			int offset = (addr & ~huge_page_mask(hs)) >> PAGE_SHIFT;
> -			pfn = huge_pte_to_pagemap_entry(*pte, offset);
> -		}
> +		/* calculate pfn of the "raw" page in the hugepage. */
> +		int offset = (addr & ~huge_page_mask(hs)) >> PAGE_SHIFT;
> +		/* next hugepage */
> +		if (!offset)
> +			pte = huge_pte_offset(mm, addr);
> +		pfn = huge_pte_to_pagemap_entry(*pte, offset);
>  		err = add_to_pagemap(addr, pfn, pm);
>  		if (err)
>  			return err;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3899395..5faafc2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -783,8 +783,8 @@ struct mm_walk {
>  	int (*pmd_entry)(pmd_t *, unsigned long, unsigned long, struct mm_walk *);
>  	int (*pte_entry)(pte_t *, unsigned long, unsigned long, struct mm_walk *);
>  	int (*pte_hole)(unsigned long, unsigned long, struct mm_walk *);
> -	int (*hugetlb_entry)(pte_t *, unsigned long, unsigned long,
> -			     struct mm_walk *);
> +	int (*hugetlb_entry)(struct vm_area_struct *,
> +			     unsigned long, unsigned long, struct mm_walk *);
>  	struct mm_struct *mm;
>  	void *private;
>  };
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 7b47a57..3148dc5 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -128,20 +128,14 @@ int walk_page_range(unsigned long addr, unsigned long end,
>  		vma = find_vma(walk->mm, addr);
>  #ifdef CONFIG_HUGETLB_PAGE
>  		if (vma && is_vm_hugetlb_page(vma)) {
> -			pte_t *pte;
> -			struct hstate *hs;
> -
>  			if (vma->vm_end < next)
>  				next = vma->vm_end;
> -			hs = hstate_vma(vma);
> -			pte = huge_pte_offset(walk->mm,
> -					      addr & huge_page_mask(hs));
> -			if (pte && !huge_pte_none(huge_ptep_get(pte))
> -			    && walk->hugetlb_entry)
> -				err = walk->hugetlb_entry(pte, addr,
> -							  next, walk);
> +			if (walk->hugetlb_entry)
> +				err = walk->hugetlb_entry(vma, addr, next,
> +							  walk);
>  			if (err)
>  				break;
> +			pgd = pgd_offset(walk->mm, next);
>  			continue;
>  		}
>  #endif
> -- 
> 1.7.0
> 
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