Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

From: Kirill A. Shutemov
Date: Thu Oct 18 2012 - 19:58:25 EST

On Thu, Oct 18, 2012 at 04:45:02PM -0700, Andrew Morton wrote:
> On Mon, 15 Oct 2012 09:00:59 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > H. Peter Anvin doesn't like huge zero page which sticks in memory forever
> > after the first allocation. Here's implementation of lockless refcounting
> > for huge zero page.
> >
> > We have two basic primitives: {get,put}_huge_zero_page(). They
> > manipulate reference counter.
> >
> > If counter is 0, get_huge_zero_page() allocates a new huge page and
> > takes two references: one for caller and one for shrinker. We free the
> > page only in shrinker callback if counter is 1 (only shrinker has the
> > reference).
> >
> > put_huge_zero_page() only decrements counter. Counter is never zero
> > in put_huge_zero_page() since shrinker holds on reference.
> >
> > Freeing huge zero page in shrinker callback helps to avoid frequent
> > allocate-free.
> I'd like more details on this please. The cost of freeing then
> reinstantiating that page is tremendous, because it has to be zeroed
> out again. If there is any way at all in which the kernel can be made
> to enter a high-frequency free/reinstantiate pattern then I expect the
> effects would be quite bad.
> Do we have sufficient mechanisms in there to prevent this from
> happening in all cases? If so, what are they, because I'm not seeing
> them?

We only free huge zero page in shrinker callback if nobody in the system
uses it. Never on put_huge_zero_page(). Shrinker runs only under memory
pressure or if user asks (drop_caches).
Do you think we need an additional protection mechanism?

> > Refcounting has cost. On 4 socket machine I observe ~1% slowdown on
> > parallel (40 processes) read page faulting comparing to lazy huge page
> > allocation. I think it's pretty reasonable for synthetic benchmark.

Kirill A. Shutemov
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