From: Andrew Morton <akpm@linux-foundation.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm@kvack.org, Andi Kleen <ak@linux.intel.com>,
"H. Peter Anvin" <hpa@linux.intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page
Date: Mon, 22 Oct 2012 23:43:49 -0700 [thread overview]
Message-ID: <20121022234349.27f33f62.akpm@linux-foundation.org> (raw)
In-Reply-To: <20121023063532.GA15870@shutemov.name>
On Tue, 23 Oct 2012 09:35:32 +0300 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
> On Fri, Oct 19, 2012 at 02:59:41AM +0300, Kirill A. Shutemov wrote:
> > 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@linux.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?
>
> Andrew?
>
Well, how hard is it to trigger the bad behavior? One can easily
create a situation in which that page's refcount frequently switches
from 0 to 1 and back again. And one can easily create a situation in
which the shrinkers are being called frequently. Run both at the same
time and what happens?
next prev parent reply other threads:[~2012-10-23 6:44 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-15 6:00 [PATCH v4 00/10, REBASED] Introduce huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 01/10] thp: huge zero page: basic preparation Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 02/10] thp: zap_huge_pmd(): zap huge zero pmd Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 03/10] thp: copy_huge_pmd(): copy huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 04/10] thp: do_huge_pmd_wp_page(): handle " Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 05/10] thp: change_huge_pmd(): keep huge zero page write-protected Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 06/10] thp: change split_huge_page_pmd() interface Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 07/10] thp: implement splitting pmd for huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 08/10] thp: setup huge zero page on non-write page fault Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 09/10] thp: lazy huge zero page allocation Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 10/10] thp: implement refcounting for huge zero page Kirill A. Shutemov
2012-10-18 23:45 ` Andrew Morton
2012-10-18 23:59 ` Kirill A. Shutemov
2012-10-23 6:35 ` Kirill A. Shutemov
2012-10-23 6:43 ` Andrew Morton [this message]
2012-10-23 7:00 ` Kirill A. Shutemov
2012-10-23 22:59 ` Andrew Morton
2012-10-23 23:38 ` Kirill A. Shutemov
2012-10-24 19:22 ` Andrew Morton
2012-10-24 19:45 ` Kirill A. Shutemov
2012-10-24 20:25 ` Andrew Morton
2012-10-24 20:33 ` Kirill A. Shutemov
2012-10-24 20:44 ` Andi Kleen
2012-10-25 20:49 ` Kirill A. Shutemov
2012-10-25 21:05 ` Andrew Morton
2012-10-25 21:22 ` Kirill A. Shutemov
2012-10-25 21:37 ` Andrew Morton
2012-10-25 22:10 ` Kirill A. Shutemov
2012-10-16 9:53 ` [PATCH v4 00/10, REBASED] Introduce " Ni zhan Chen
2012-10-16 10:54 ` Kirill A. Shutemov
2012-10-16 11:13 ` Ni zhan Chen
2012-10-16 11:28 ` Kirill A. Shutemov
2012-10-16 11:37 ` Ni zhan Chen
2012-10-26 15:14 ` [PATCH] thp, vmstat: implement HZP_ALLOC and HZP_ALLOC_FAILED events Kirill A. Shutemov
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