Re: [PATCH v2 2/2] mm: don't use compound_head() in virt_to_head_page()

[Andrew Morton]

On Thu, 15 Jan 2015 16:40:33 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:

> compound_head() is implemented with assumption that there would be
> race condition when checking tail flag. This assumption is only true
> when we try to access arbitrary positioned struct page.
> 
> The situation that virt_to_head_page() is called is different case.
> We call virt_to_head_page() only in the range of allocated pages,
> so there is no race condition on tail flag. In this case, we don't
> need to handle race condition and we can reduce overhead slightly.
> This patch implements compound_head_fast() which is similar with
> compound_head() except tail flag race handling. And then,
> virt_to_head_page() uses this optimized function to improve performance.
> 
> I saw 1.8% win in a fast-path loop over kmem_cache_alloc/free,
> (14.063 ns -> 13.810 ns) if target object is on tail page.
>
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -453,6 +453,13 @@ static inline struct page *compound_head(struct page *page)
>  	return page;
>  }
>  
> +static inline struct page *compound_head_fast(struct page *page)
> +{
> +	if (unlikely(PageTail(page)))
> +		return page->first_page;
> +	return page;
> +}

Can we please have some code comments which let people know when they
should and shouldn't use compound_head_fast()?  I shouldn't have to say
this :(

>  /*
>   * The atomic page->_mapcount, starts from -1: so that transitions
>   * both from it and to it can be tracked, using atomic_inc_and_test
> @@ -531,7 +538,8 @@ static inline void get_page(struct page *page)
>  static inline struct page *virt_to_head_page(const void *x)
>  {
>  	struct page *page = virt_to_page(x);
> -	return compound_head(page);
> +
> +	return compound_head_fast(page);

And perhaps some explanation here as to why virt_to_head_page() can
safely use compound_head_fast().  There's an assumption here that
nobody will be dismantling the compound page while virt_to_head_page()
is in progress, yes?  And this assumption also holds for the calling
code, because otherwise the virt_to_head_page() return value is kinda
meaningless.

This is tricky stuff - let's spell it out carefully.
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