On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <[email protected]>
>
> 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.
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