linux-kernel - Re: [PATCH] mm: Move __vma_address() to internal.h to be inlined in huge

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Date:	Mon, 16 Jun 2014 15:34:47 -0400
From:	Waiman Long <waiman.long@...com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH] mm: Move __vma_address() to internal.h to be inlined
 in huge_memory.c

On 06/12/2014 05:45 PM, David Rientjes wrote:
> On Thu, 12 Jun 2014, Waiman Long wrote:
>
>>>> The vma_address() function which is used to compute the virtual address
>>>> within a VMA is used only by 2 files in the mm subsystem - rmap.c and
>>>> huge_memory.c. This function is defined in rmap.c and is inlined by
>>>> its callers there, but it is also declared as an external function.
>>>>
>>>> However, the __split_huge_page() function which calls vma_address()
>>>> in huge_memory.c is calling it as a real function call. This is not
>>>> as efficient as an inlined function. This patch moves the underlying
>>>> inlined __vma_address() function to internal.h to be shared by both
>>>> the rmap.c and huge_memory.c file.
>>> This increases huge_memory.o's text+data_bss by 311 bytes, which makes
>>> me suspect that it is a bad change due to its increase of kernel cache
>>> footprint.
>>>
>>> Perhaps we should be noinlining __vma_address()?
>> On my test machine, I saw an increase of 144 bytes in the text segment
>> of huge_memory.o. The size in size is caused by an increase in the size
>> of the __split_huge_page function. When I remove the
>>
>>          if (unlikely(is_vm_hugetlb_page(vma)))
>>                  pgoff = page->index<<  huge_page_order(page_hstate(page));
>>
>> check, the increase in size drops down to 24 bytes. As a THP cannot be
>> a hugetlb page, there is no point in doing this check for a THP. I will
>> update the patch to pass in an additional argument to disable this
>> check for __split_huge_page.
>>
> I think we're seeking a reason or performance numbers that suggest
> __vma_address() being inline is appropriate and so far we lack any such
> evidence.  Adding additional parameters to determine checks isn't going to
> change the fact that it increases text size needlessly.

This patch was motivated by my investigation of a freeze problem of an 
application running on SLES11 sp3 which was caused by the long time it 
took to munmap part of a THP. Inlining vma_address help a bit in that 
situation. However, the problem will be essentially gone after including 
patches that changing the anon_vma_chain to use rbtree instead of a 
simple list.

I do agree that performance impact of inlining vma_address in minimal in 
the latest kernel. So I am not going to pursue this any further.

Thank for the review.

-Longman
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