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

[Vlastimil Babka]

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...


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