linux-kernel - Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

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Date:	Fri, 28 May 2010 18:34:13 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Doug Doan <dougd@...y.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"lee.schermerhorn@...com" <lee.schermerhorn@...com>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

On Fri, May 28, 2010 at 09:39:59AM -0700, Doug Doan wrote:
> On 05/28/2010 02:59 AM, Mel Gorman wrote:
>> On Thu, May 27, 2010 at 01:43:00PM -0700, Doug Doan wrote:
>>> From: Doug Doan<dougd@...y.com>
>>>
>>> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
>>> through handle_pte_fault for normal pages, or through hugetlb_fault for
>>> huge pages.
>>>
>>> In the normal page case, we eventually get to do_wp_page and call mmu
>>> notifiers via ptep_clear_flush_notify. There is no callout to the mmmu
>>> notifiers in the huge page case. This patch fixes that.
>>>
>>> Signed-off-by: Doug Doan<dougd@...y.com>
>>> ---
>>
>>> --- mm/hugetlb.c.orig	2010-05-27 13:07:58.569546314 -0700
>>> +++ mm/hugetlb.c	2010-05-26 14:41:06.449296524 -0700
>>> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
>>>   	ptep = huge_pte_offset(mm, address&  huge_page_mask(h));
>>>   	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
>>>   		/* Break COW */
>>> +		mmu_notifier_invalidate_range_start(mm,
>>> +			address&  huge_page_mask(h),
>>> +			(address&  huge_page_mask(h)) + huge_page_size(h));
>>
>> Should the address not already be aligned?
>
> I'm not seeing where the address was aligned before this point. The code 
> just above aligns it:
>
>    	ptep = huge_pte_offset(mm, address&  huge_page_mask(h));
>    	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
>    		/* Break COW */
>

I'm sorry. You're right. I was looking at the copy_huge_page which was
not aligning the address. It should be but it's ultimately harmless as
the parameter is discarded.

Acked-by: Mel Gorman <mel@....ul.ie>

>> Otherwise, I don't see any problem.
>>
>>>   		huge_ptep_clear_flush(vma, address, ptep);
>>>   		set_huge_pte_at(mm, address, ptep,
>>>   				make_huge_pte(vma, new_page, 1));
>>>   		/* Make the old page be freed below */
>>>   		new_page = old_page;
>>> +		mmu_notifier_invalidate_range_end(mm,
>>> +			address&  huge_page_mask(h),
>>> +			(address&  huge_page_mask(h)) + huge_page_size(h));
>>>   	}
>>>   	page_cache_release(new_page);
>>>   	page_cache_release(old_page);
>>
>>
>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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