linux-kernel - Re: mm: slub: invalid memory access in setup

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Message-ID: <20140702020454.GA6961@richard>
Date:	Wed, 2 Jul 2014 10:04:54 +0800
From:	Wei Yang <weiyang@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: mm: slub: invalid memory access in setup_object

On Tue, Jul 01, 2014 at 09:58:52AM -0500, Christoph Lameter wrote:
>On Mon, 30 Jun 2014, David Rientjes wrote:
>
>> It's not at all clear to me that that patch is correct.  Wei?
>
>Looks ok to me. But I do not like the convoluted code in new_slab() which
>Wei's patch does not make easier to read. Makes it difficult for the
>reader to see whats going on.

My patch is somewhat convoluted since I wanted to preserve the original logic
and make minimal change. And yes, it looks not that nice to audience.

I feel a little hurt by this patch. What I found and worked is gone with this
patch.

>
>Lets drop the use of the variable named "last".
>
>
>Subject: slub: Only call setup_object once for each object
>
>Modify the logic for object initialization to be less convoluted
>and initialize an object only once.
>
>Signed-off-by: Christoph Lameter <cl@...ux.com>
>
>Index: linux/mm/slub.c
>===================================================================
>--- linux.orig/mm/slub.c	2014-07-01 09:50:02.486846653 -0500
>+++ linux/mm/slub.c	2014-07-01 09:52:07.918802585 -0500
>@@ -1409,7 +1409,6 @@ static struct page *new_slab(struct kmem
> {
> 	struct page *page;
> 	void *start;
>-	void *last;
> 	void *p;
> 	int order;
>
>@@ -1432,15 +1431,11 @@ static struct page *new_slab(struct kmem
> 	if (unlikely(s->flags & SLAB_POISON))
> 		memset(start, POISON_INUSE, PAGE_SIZE << order);
>
>-	last = start;
> 	for_each_object(p, s, start, page->objects) {
>-		setup_object(s, page, last);
>-		set_freepointer(s, last, p);
>-		last = p;
>+		setup_object(s, page, p);
>+		set_freepointer(s, p, p + s->size);
> 	}
>-	setup_object(s, page, last);
>-	set_freepointer(s, last, NULL);
>-
>+	set_freepointer(s, start + (page->objects - 1) * s->size, NULL);
> 	page->freelist = start;
> 	page->inuse = page->objects;
> 	page->frozen = 1;

-- 
Richard Yang
Help you, Help me

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