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

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Message-ID: <4C06E5A6.6@cray.com>
Date:	Wed, 2 Jun 2010 16:13:42 -0700
From:	Doug Doan <dougd@...y.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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>,
	"mel@....ul.ie" <mel@....ul.ie>,
	Andrea Arcangeli <andrea@...hat.com>
Subject: Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

On 06/01/2010 11:16 PM, Andrew Morton wrote:
> On Thu, 27 May 2010 13:43:00 -0700 Doug Doan<dougd@...y.com>  wrote:
>
>>
>> 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>
>> ---
>>
>> [patch  text/plain (802B)]
>> --- mm/hugetlb.c.orig	2010-05-27 13:07:58.569546314 -0700
>> +++ mm/hugetlb.c	2010-05-26 14:41:06.449296524 -0700
>
> (In patch -p1 form, please.  So a/mm/hugetlb.c)
>
>> @@ -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));
>>   		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);
>
> This causes mmu_notifier_invalidate_range_start() to be called under
> page_table_lock.  The immediately preceding code seems to take some
> care to avoid doing that.  I took a quick look at other callsites and
> cannot immediately see other cases where
> mmu_notifier_invalidate_range_start/end() are called under that lock.
>
> This may not introduce bugs with current notifier implementations (I
> didn't check), but it does lessen flexibility?

In the normal page case, handle_pte_fault calls do_wp_page inside a spinlock on 
ptl = pte_lockptr(mm, pmd), which uses mm->page_table_lock if USE_SPLIT_PTLOCKS 
is not defined.

I don't understand what you mean by lessen flexibilty.
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