Re: [RFC V2] mm:change smaps/pagemap_read calculation behavior

[Naoya Horiguchi]

On Fri, Jan 30, 2015 at 03:47:54PM +0800, Wang, Yalin wrote:
> This patch change smaps/pagemap_read pagetable walk behavior, to make
> sure not skip VM_PFNMAP pagetables,
> so that we can calculate COW pages of VM_PFNMAP as normal pages.
> 
> Signed-off-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx>

Hi Yalin,

The original motivation of the VM_PFNMAP code in pagewalk.c comes from the
following patch:

  commit a9ff785e4437c83d2179161e012f5bdfbd6381f0
  Author: Cliff Wickman <cpw@xxxxxxx>
  Date:   Fri May 24 15:55:36 2013 -0700
  
      mm/pagewalk.c: walk_page_range should avoid VM_PFNMAP areas

, where Cliff stated that some kind of vma(VM_PFNMAP) caused kernel panic
when walk_page_range() was called over it. So I don't think that re-enabling
to walk over every vma(VM_PFNMAP) unexceptionally is a good idea.

If you really want to get some information from a vma(VM_PFNMAP) via these
interfaces, I recommend you to implement proper judging code which returns
0 for your vma(VM_PFNMAP) and returns 1 for Cliff's vma(VM_PFNMAP).

Thanks,
Naoya Horiguchi

> ---
>  fs/proc/task_mmu.c | 2 ++
>  include/linux/mm.h | 2 ++
>  mm/pagewalk.c      | 5 +++++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c7267e9..e7d7c43 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -616,6 +616,7 @@ static int show_smap(struct seq_file *m, void *v, int is_pid)
>  	struct mem_size_stats mss;
>  	struct mm_walk smaps_walk = {
>  		.pmd_entry = smaps_pte_range,
> +		.test_walk = generic_walk_page_test_no_skip,
>  		.mm = vma->vm_mm,
>  		.private = &mss,
>  	};
> @@ -1264,6 +1265,7 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>  
>  	pagemap_walk.pmd_entry = pagemap_pte_range;
>  	pagemap_walk.pte_hole = pagemap_pte_hole;
> +	pagemap_walk.test_walk = generic_walk_page_test_no_skip;
>  #ifdef CONFIG_HUGETLB_PAGE
>  	pagemap_walk.hugetlb_entry = pagemap_hugetlb_range;
>  #endif
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b976d9f..07f71c5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1191,6 +1191,8 @@ struct mm_walk {
>  	void *private;
>  };
>  
> +int generic_walk_page_test_no_skip(unsigned long start, unsigned long end,
> +		struct mm_walk *walk);
>  int walk_page_range(unsigned long addr, unsigned long end,
>  		struct mm_walk *walk);
>  int walk_page_vma(struct vm_area_struct *vma, struct mm_walk *walk);
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 75c1f28..14f38d5 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -206,6 +206,11 @@ static int __walk_page_range(unsigned long start, unsigned long end,
>  	return err;
>  }
>  
> +int generic_walk_page_test_no_skip(unsigned long start, unsigned long end,
> +			struct mm_walk *walk)
> +{
> +	return 0;
> +}
>  /**
>   * walk_page_range - walk page table with caller specific callbacks
>   *
> -- 
> 2.2.2
> --
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