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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 19 Oct 2012 02:59:41 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

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@...ux.intel.com> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/