Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the page on WP fault

[Vlastimil Babka]

On 05/15/2015 03:29 PM, Kirill A. Shutemov wrote:
> On Fri, May 15, 2015 at 01:35:49PM +0200, Vlastimil Babka wrote:
> > On 05/15/2015 01:21 PM, Kirill A. Shutemov wrote:
> > > On Fri, May 15, 2015 at 11:15:00AM +0200, Vlastimil Babka wrote:
> > > > On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> > > > > With new refcounting we will be able map the same compound page with
> > > > > PTEs and PMDs. It requires adjustment to conditions when we can reuse
> > > > > the page on write-protection fault.
> > > > >
> > > > > For PTE fault we can't reuse the page if it's part of huge page.
> > > > >
> > > > > For PMD we can only reuse the page if nobody else maps the huge page or
> > > > > it's part. We can do it by checking page_mapcount() on each sub-page,
> > > > > but it's expensive.
> > > > >
> > > > > The cheaper way is to check page_count() to be equal 1: every mapcount
> > > > > takes page reference, so this way we can guarantee, that the PMD is the
> > > > > only mapping.
> > > > >
> > > > > This approach can give false negative if somebody pinned the page, but
> > > > > that doesn't affect correctness.
> > > > >
> > > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > > Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > > >
> > > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> > > >
> > > > So couldn't the same trick be used in Patch 1 to avoid counting individual
> > > > oder-0 pages?
> > >
> > > Hm. You're right, we could. But is smaps that performance sensitive to
> > > bother?
> >
> > Well, I was nudged to optimize it when doing the shmem swap accounting
> > changes there :) User may not care about the latency of obtaining the smaps
> > file contents, but since it has mmap_sem locked for that, the process might
> > care...
>
> Somewthing like this?

Yeah, that should work.

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index e04399e53965..5bc3d2b1176e 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -462,6 +462,19 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>          if (young || PageReferenced(page))
>                  mss->referenced += size;
>
> +       /*
> +        * page_count(page) == 1 guarantees the page is mapped exactly once.
> +        * If any subpage of the compound page mapped with PTE it would elevate
> +        * page_count().
> +        */
> +       if (page_count(page) == 1) {
> +               if (dirty || PageDirty(page))
> +                       mss->private_dirty += size;
> +               else
> +                       mss->private_clean += size;
> +               return;
> +       }
> +
>          for (i = 0; i < nr; i++, page++) {
>                  int mapcount = page_mapcount(page);

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