From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrew Morton <akpm@linux-foundation.org>
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: Wed, 24 Oct 2012 02:38:01 +0300 [thread overview]
Message-ID: <20121023233801.GA21591@shutemov.name> (raw)
In-Reply-To: <20121023155915.7d5ef9d1.akpm@linux-foundation.org>
On Tue, Oct 23, 2012 at 03:59:15PM -0700, Andrew Morton wrote:
> On Tue, 23 Oct 2012 10:00:18 +0300
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
>
> > > 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?
> >
> > If the goal is to trigger bad behavior then:
> >
> > 1. read from an area where a huge page can be mapped to get huge zero page
> > mapped. hzp is allocated here. refcounter == 2.
> > 2. write to the same page. refcounter == 1.
> > 3. echo 3 > /proc/sys/vm/drop_caches. refcounter == 0 -> free the hzp.
> > 4. goto 1.
> >
> > But it's unrealistic. /proc/sys/vm/drop_caches is only root-accessible.
>
> Yes, drop_caches is uninteresting.
>
> > We can trigger shrinker only under memory pressure. But in this, most
> > likely we will get -ENOMEM on hzp allocation and will go to fallback path
> > (4k zero page).
>
> I disagree. If, for example, there is a large amount of clean
> pagecache being generated then the shrinkers will be called frequently
> and memory reclaim will be running at a 100% success rate. The
> hugepage allocation will be successful in such a situation?
Yes.
Shrinker callbacks are called from shrink_slab() which happens after page
cache reclaim, so on next reclaim round page cache will reclaim first and
we will avoid frequent alloc-free pattern.
One more thing we can do: increase shrinker->seeks to something like
DEFAULT_SEEKS * 4. In this case shrink_slab() will call our callback after
callbacks with DEFAULT_SEEKS.
--
Kirill A. Shutemov
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WARNING: multiple messages have this Message-ID (diff)
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrew Morton <akpm@linux-foundation.org>
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: Wed, 24 Oct 2012 02:38:01 +0300 [thread overview]
Message-ID: <20121023233801.GA21591@shutemov.name> (raw)
In-Reply-To: <20121023155915.7d5ef9d1.akpm@linux-foundation.org>
On Tue, Oct 23, 2012 at 03:59:15PM -0700, Andrew Morton wrote:
> On Tue, 23 Oct 2012 10:00:18 +0300
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
>
> > > 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?
> >
> > If the goal is to trigger bad behavior then:
> >
> > 1. read from an area where a huge page can be mapped to get huge zero page
> > mapped. hzp is allocated here. refcounter == 2.
> > 2. write to the same page. refcounter == 1.
> > 3. echo 3 > /proc/sys/vm/drop_caches. refcounter == 0 -> free the hzp.
> > 4. goto 1.
> >
> > But it's unrealistic. /proc/sys/vm/drop_caches is only root-accessible.
>
> Yes, drop_caches is uninteresting.
>
> > We can trigger shrinker only under memory pressure. But in this, most
> > likely we will get -ENOMEM on hzp allocation and will go to fallback path
> > (4k zero page).
>
> I disagree. If, for example, there is a large amount of clean
> pagecache being generated then the shrinkers will be called frequently
> and memory reclaim will be running at a 100% success rate. The
> hugepage allocation will be successful in such a situation?
Yes.
Shrinker callbacks are called from shrink_slab() which happens after page
cache reclaim, so on next reclaim round page cache will reclaim first and
we will avoid frequent alloc-free pattern.
One more thing we can do: increase shrinker->seeks to something like
DEFAULT_SEEKS * 4. In this case shrink_slab() will call our callback after
callbacks with DEFAULT_SEEKS.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2012-10-23 23:36 UTC|newest]
Thread overview: 63+ 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 ` 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 ` 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 ` 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 ` 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 ` 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 ` 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 ` 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 ` 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 ` 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 ` Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 10/10] thp: implement refcounting for huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` Kirill A. Shutemov
2012-10-18 23:45 ` Andrew Morton
2012-10-18 23:45 ` Andrew Morton
2012-10-18 23:59 ` Kirill A. Shutemov
2012-10-18 23:59 ` Kirill A. Shutemov
2012-10-23 6:35 ` Kirill A. Shutemov
2012-10-23 6:35 ` Kirill A. Shutemov
2012-10-23 6:43 ` Andrew Morton
2012-10-23 6:43 ` Andrew Morton
2012-10-23 7:00 ` Kirill A. Shutemov
2012-10-23 22:59 ` Andrew Morton
2012-10-23 22:59 ` Andrew Morton
2012-10-23 23:38 ` Kirill A. Shutemov [this message]
2012-10-23 23:38 ` Kirill A. Shutemov
2012-10-24 19:22 ` Andrew Morton
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 20:25 ` Andrew Morton
2012-10-24 20:33 ` Kirill A. Shutemov
2012-10-24 20:44 ` Andi Kleen
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 21:05 ` Andrew Morton
2012-10-25 21:22 ` Kirill A. Shutemov
2012-10-25 21:22 ` Kirill A. Shutemov
2012-10-25 21:37 ` Andrew Morton
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 9:53 ` Ni zhan Chen
2012-10-16 10:54 ` Kirill A. Shutemov
2012-10-16 10:54 ` Kirill A. Shutemov
2012-10-16 11:13 ` Ni zhan Chen
2012-10-16 11:13 ` Ni zhan Chen
2012-10-16 11:28 ` Kirill A. Shutemov
2012-10-16 11:28 ` Kirill A. Shutemov
2012-10-16 11:37 ` Ni zhan Chen
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
2012-10-26 15:14 ` Kirill A. Shutemov
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