linux-kernel - Re: [PATCH] mm: mmu_notifier: re-fix freed page still mapped in secondary MMU

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Date:	Tue, 16 Apr 2013 18:26:36 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Robin Holt <holt@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: mmu_notifier: re-fix freed page still mapped in secondary
 MMU

On 04/16/2013 05:31 PM, Robin Holt wrote:
> On Tue, Apr 16, 2013 at 02:39:49PM +0800, Xiao Guangrong wrote:
>> The commit 751efd8610d3 (mmu_notifier_unregister NULL Pointer deref
>> and multiple ->release()) breaks the fix:
>>     3ad3d901bbcfb15a5e4690e55350db0899095a68
>>     (mm: mmu_notifier: fix freed page still mapped in secondary MMU)
> 
> Can you describe how the page is still mapped?  I thought I had all
> cases covered.  Whichever call hits first, I thought we had one callout
> to the registered notifiers.  Are you saying we need multiple callouts?

No.

You patch did this:

                hlist_del_init_rcu(&mn->hlist);    1 <======
+               spin_unlock(&mm->mmu_notifier_mm->lock);
+
+               /*
+                * Clear sptes. (see 'release' description in mmu_notifier.h)
+                */
+               if (mn->ops->release)
+                       mn->ops->release(mn, mm);    2 <======
+
+               spin_lock(&mm->mmu_notifier_mm->lock);

At point 1, you delete the notify, but the page is still on LRU. Other
cpu can reclaim the page but without call ->invalid_page().

At point 2, you call ->release(), the secondary MMU make page Accessed/Dirty
but that page has already been on the free-list of page-alloctor.

> 
> Also, shouldn't you be asking for a revert commit and then supply a
> subsequent commit for the real fix?  I thought that was the process for
> doing a revert.

Can not do that pure reversion since your patch moved hlist_for_each_entry_rcu
which has been modified now.

Should i do pure-eversion + hlist_for_each_entry_rcu update first?

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