linux-kernel - Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE

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Message-ID: <556D7851.1020107@suse.cz>
Date:	Tue, 02 Jun 2015 11:33:05 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE_PAGES

On 06/02/2015 11:25 AM, Michal Hocko wrote:
> On Mon 25-05-15 17:24:28, Vlastimil Babka wrote:
>> On 05/22/2015 04:35 PM, Mel Gorman wrote:
>>>>
>>>> Thanks!
>>>>
>>>>> This makes a lot of sense to me.  The only thing I worry about is the
>>>>> proliferation of PageHuge(), a function call, in relatively hot paths.
>>>>
>>>> I've tried that (see the patch below) but it enlarged the code by almost
>>>> 1k
>>>>     text    data     bss     dec     hex filename
>>>>   510323   74273   44440  629036   9992c mm/built-in.o.before
>>>>   511248   74273   44440  629961   99cc9 mm/built-in.o.after
>>>>
>>>> I am not sure the code size increase is worth it. Maybe we can reduce
>>>> the check to only PageCompound(page) as huge pages are no in the page
>>>> cache (yet).
>>>>
>>>
>>> That would be a more sensible route because it also avoids exposing the
>>> hugetlbfs destructor unnecessarily.
>>
>> You could maybe do test such as (PageCompound(page) && PageHuge(page)) to
>> short-circuit the call while remaining future-proof.
>
> How about this?

Yeah (see below)

> ---
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 91b7f9b2b774..bb8a70e8fc77 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -547,7 +547,13 @@ static inline void ClearPageCompound(struct page *page)
>   #endif /* !PAGEFLAGS_EXTENDED */
>
>   #ifdef CONFIG_HUGETLB_PAGE
> -int PageHuge(struct page *page);
> +int __PageHuge(struct page *page);
> +static inline int PageHuge(struct page *page)
> +{
> +	if (!PageCompound(page))

Perhaps the above as likely()?

[...]

> -EXPORT_SYMBOL_GPL(PageHuge);
> +EXPORT_SYMBOL_GPL(__PageHuge);
>
>   /*
>    * PageHeadHuge() only returns true for hugetlbfs head page, but not for
>

Do the same thing here by inlining the PageHead() test?
I guess the page_to_pgoff and __compound_tail_refcounted callers are 
rather hot?


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