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Subject: Re: [PATCH] mm: Fix potential pte_unmap_unlock pte error
To:     Andi Kleen <ak@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     <tglx@linutronix.de>, <dave.hansen@intel.com>,
        <jpoimboe@redhat.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
References: <20210109080118.20885-1-linmiaohe@huawei.com>
 <20210110171443.GC1914459@tassilo.jf.intel.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <530deddf-705e-045d-f7c6-521531dced71@huawei.com>
Date:   Thu, 14 Jan 2021 10:51:24 +0800
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Hi:
On 2021/1/11 1:14, Andi Kleen wrote:
> On Sat, Jan 09, 2021 at 03:01:18AM -0500, Miaohe Lin wrote:
>> Since commit 42e4089c7890 ("x86/speculation/l1tf: Disallow non privileged
>> high MMIO PROT_NONE mappings"), when the first pfn modify is not allowed,
>> we would break the loop with pte unchanged. Then the wrong pte - 1 would
>> be passed to pte_unmap_unlock.
> 
> Thanks.
> 
> While the fix is correct, I'm not sure if it actually is a real bug. Is there
> any architecture that would do something else than unlocking the underlying
> page?  If it's just the underlying page then it should be always the same
> page, so no bug.
> 

It's just a theoretical issue via code inspection.

> That said of course the change is the right thing for main line, but probably doesn't
> need to be backported.
> 

So it should not be backported. Should I resend a patch or Andrew would kindly do this?

> -Andi
> .
> 

Many thanks for review and reply.