Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE_PAGES

Vlastimil Babka
2015-06-02T09:33:13Z

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?