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Date: Tue, 20 Aug 2013 17:18:16 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 1/4] mm/pgtable: Fix continue to preallocate pmds
 even if failure occurrence
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To: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, Rik van Riel <riel@redhat.com>, Fengguang Wu <fengguang.wu@intel.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>, Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>, David Rientjes <rientjes@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Jiri Kosina <jkosina@suse.cz>, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On Wed, 21 Aug 2013 07:39:35 +0800 Wanpeng Li <liwanp@linux.vnet.ibm.com> wrote:

> >Nope.  If the error path is taken, free_pmds() will free uninitialised
> >items from pmds[], which is a local in pgd_alloc() and contains random
> >stack junk.  The kernel will crash.
> >
> >You could pass an nr_pmds argument to free_pmds(), or zero out the
> >remaining items on the error path.  However, although the current code
> >is a bit kooky, I don't see that it is harmful in any way.
> >
> 
> There is a check in free_pmds():
> 
> if (pmds[i])
> 	free_page((unsigned long)pmds[i]);
> 
> which will avoid the issue you mentioned.

pmds[i] is uninitialized.  It gets allocated
on the stack in pgd_alloc() and does not get zeroed.

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