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

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Date:	Thu, 12 Jun 2014 17:34:18 -0400
From:	Waiman Long <waiman.long@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	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 03:25 PM, Andrew Morton wrote:
> On Thu, 12 Jun 2014 15:15:40 -0400 Waiman Long<Waiman.Long@...com>  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.

-Longman

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