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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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?