From: Andrea Arcangeli <aarcange@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
kosaki.motohiro@jp.fujitsu.com,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [BUGFIX][PATCH] Fix false positive BUG_ON in __page_set_anon_rmap
Date: Thu, 22 Jul 2010 19:39:20 +0200 [thread overview]
Message-ID: <20100722173920.GI24928@random.random> (raw)
In-Reply-To: <4C4844BC.4090709@redhat.com>
On Thu, Jul 22, 2010 at 09:16:44AM -0400, Rik van Riel wrote:
> On 07/22/2010 03:41 AM, KAMEZAWA Hiroyuki wrote:
> > Rik, how do you think ?
> >
> > ==
> > From: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
> >
> > Problem: wrong BUG_ON() in __page_set_anon_rmap().
> > Kernel version: mmotm-0719
>
> > Description:
> > Even if SwapCache is fully unmapped and mapcount goes down to 0,
> > page->mapping is not cleared and will remain on memory until kswapd or some
> > finds it. If a thread cause a page fault onto such "unmapped-but-not-discarded"
> > swapcache, it will see a swap cache whose mapcount is 0 but page->mapping has a
> > valid value.
> >
> > When it's reused at do_swap_page(), __page_set_anon_rmap() is called with
> > "exclusive==1" and hits BUG_ON(). But this BUG_ON() is wrong. Nothing bad
> > with rmapping a page which has page->mapping isn't 0.
>
> Yes, you are absolutely right.
>
I already noticed the problem when I merged your patch in aa.git
(before it would only be exclusive=0 in do_swap_page so it wasn't a
false positive), and I fixed it this way:
http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=commitdiff;h=2fe4f42f0f17498984b3f86b2339d583004b45de;hp=ffd146080305632406d97c7f6f984a648854d755
So I retained the BUG_ON for the real page_add_anon_rmap. Maybe not
worth it but you can have a look at my solution if you're interested
to retain it too.
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prev parent reply other threads:[~2010-07-22 17:39 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-22 7:41 [BUGFIX][PATCH] Fix false positive BUG_ON in __page_set_anon_rmap KAMEZAWA Hiroyuki
2010-07-22 13:16 ` Rik van Riel
2010-07-22 17:39 ` Andrea Arcangeli [this message]
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