linux-kernel - [PATCH] xen: fix scrub

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Message-ID: <20081117181422.GA8321@elte.hu>
Date:	Mon, 17 Nov 2008 19:14:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Tejun Heo <htejun@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: [PATCH] xen: fix scrub_page()


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

>> Secondly, we actually have a function called "clear_highpage()" 
>> that does this, except it uses kmap_atomic(page, KM_USER0). Which 
>> is _probably better anyway, but I didn't check if there is some 
>> magical reason why it wouldn't work.
>
> OK.

applied it with the changelog below to tip/x86/urgent, thanks guys! 

AFAICS CONFIG_XEN_SCRUB_PAGES=y could not possibly have worked before 
(kunmapping the wrong address is a crasher), so there's little 
practical risk from using an atomic kmap here.

	Ingo

-------------->
>From 26a3e99160cfb06a0a33e25b9fb0d516e2cc680d Mon Sep 17 00:00:00 2001
From: Jeremy Fitzhardinge <jeremy@...p.org>
Date: Mon, 17 Nov 2008 09:35:00 -0800
Subject: [PATCH] xen: fix scrub_page()

Impact: fix guest kernel crash with CONFIG_XEN_SCRUB_PAGES=y

Jens noticed that scrub_page() has a buggy unmap of the wrong
thing. (virtual address instead of page)

Linus pointed out that the whole scrub_page() code is an unnecessary
reimplementation of clear_highpage() to begin with.

Just use clear_highpage() rather than reimplementing it poorly.

Reported-by: Jens Axboe <jens.axboe@...cle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 drivers/xen/balloon.c |    9 +--------
 1 files changed, 1 insertions(+), 8 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index a0fb5ea..526c191 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -122,14 +122,7 @@ static struct timer_list balloon_timer;
 static void scrub_page(struct page *page)
 {
 #ifdef CONFIG_XEN_SCRUB_PAGES
-	if (PageHighMem(page)) {
-		void *v = kmap(page);
-		clear_page(v);
-		kunmap(v);
-	} else {
-		void *v = page_address(page);
-		clear_page(v);
-	}
+	clear_highpage(page);
 #endif
 }
 

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