linux-kernel - Re: [patch -mm 1/2] i386: add ptep_test_and_clear

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Message-ID: <46082B05.9050602@vmware.com>
Date:	Mon, 26 Mar 2007 12:20:21 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young}

Hugh Dickins wrote:
> On Sun, 25 Mar 2007, Zachary Amsden wrote:
>   
>> If you actually clear the bit, you need to:
>>
>> +         pte_update_defer(vma->vm_mm, addr, ptep);
>>
>> The reason is, when updating PTEs, the hypervisor must be notified.  Using
>> atomic operations to do this is fine for all hypervisors I am aware of.
>> However, for hypervisors which shadow page tables, if these PTE modifications
>> are not trapped, you need a post-modification call to fulfill the update of
>> the shadow page table.
>>     
>
> Thanks for the very rapid response.
>
> So, David just needs to move the pte_update_defer out of
> ptep_clear_flush_* and into ptep_test_and_clear_*?
>   

Yes.

> That leaves me wondering why you deleted ptep_test_and_clear_*
> (while leaving their __HAVE_ARCHes) in the first place?
>   

Because raw use of them in the arch independent MM code would introduce 
exactly this bug on i386, so leaving __HAVE_ARCH but leaving out the 
definition would catch this case.

Zach
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