Re: [PATCHv4 17/39] thp, mm: handle tail pages in page_cache_get_speculative()

[Dave Hansen]

On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> For tail page we call __get_page_tail(). It has the same semantics, but
> for tail page.

page_cache_get_speculative() has a ~50-line comment above it with lots
of scariness about grace periods and RCU.  A two line comment saying
that the semantics are the same doesn't make me feel great that you've
done your homework here.

Are there any performance implications here?  __get_page_tail() says:
"It implements the slow path of get_page().".
page_cache_get_speculative() seems awfully speculative which would make
me think that it is part of a _fast_ path.

> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 28597ec..2e86251 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -161,6 +161,9 @@ static inline int page_cache_get_speculative(struct page *page)
>  {
>  	VM_BUG_ON(in_interrupt());
>  
> +	if (unlikely(PageTail(page)))
> +		return __get_page_tail(page);
> +
>  #ifdef CONFIG_TINY_RCU
>  # ifdef CONFIG_PREEMPT_COUNT
>  	VM_BUG_ON(!in_atomic());
> @@ -187,7 +190,6 @@ static inline int page_cache_get_speculative(struct page *page)
>  		return 0;
>  	}
>  #endif
> -	VM_BUG_ON(PageTail(page));
>  
>  	return 1;
>  }

FWIW, that VM_BUG_ON() should theoretically be able to stay there since
it's unreachable now that you've short-circuited the function for
PageTail() pages.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/