Re: [PATCH 4/8] mm: FOLL_DUMP replace FOLL_ANON

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Hugh.

On Sun, 13 Sep 2009 16:46:12 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> On Thu, 10 Sep 2009, Minchan Kim wrote:
> > > A  A  A  A /*
> > > A  A  A  A  * When core dumping an enormous anonymous area that nobody
> > > - A  A  A  A * has touched so far, we don't want to allocate page tables.
> > > + A  A  A  A * has touched so far, we don't want to allocate unnecessary pages or
> > > + A  A  A  A * page tables. A Return error instead of NULL to skip handle_mm_fault,
> > > + A  A  A  A * then get_dump_page() will return NULL to leave a hole in the dump.
> > > + A  A  A  A * But we can only make this optimization where a hole would surely
> > > + A  A  A  A * be zero-filled if handle_mm_fault() actually did handle it.
> > > A  A  A  A  */
> > > - A  A  A  if (flags & FOLL_ANON) {
> > > - A  A  A  A  A  A  A  page = ZERO_PAGE(0);
> > > - A  A  A  A  A  A  A  if (flags & FOLL_GET)
> > > - A  A  A  A  A  A  A  A  A  A  A  get_page(page);
> > > - A  A  A  A  A  A  A  BUG_ON(flags & FOLL_WRITE);
> > > - A  A  A  }
> > > + A  A  A  if ((flags & FOLL_DUMP) &&
> > > + A  A  A  A  A  (!vma->vm_ops || !vma->vm_ops->fault))
> > 
> > How about adding comment about zero page use?
> 
> What kind of comment did you have in mind?
> We used to use ZERO_PAGE there, but with this patch we're not using it.
> I thought the comment above describes what we're doing well enough.
> 
> I may have kept too quiet about ZERO_PAGEs, knowing that a later patch
> was going to change the story; but I don't see what needs saying here.

I meant following as line. 

> > > + A  A  A  if ((flags & FOLL_DUMP) &&
> > > + A  A  A  A  A  (!vma->vm_ops || !vma->vm_ops->fault))

Why do we care about anonymous vma and FOLL_DUMP?
Yeah, comment above mentioned page tables. 
But i think someone who first look at can't think it easily. 

If you think the comment is enough, I don't mind it. 

> Hugh


-- 
Kind regards,
Minchan Kim

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