linux-kernel - Re: [PATCHv4 17/39] thp, mm: handle tail pages in page_cache_get

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Message-ID: <519BDDEF.9020705@sr71.net>
Date:	Tue, 21 May 2013 13:49:51 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 17/39] thp, mm: handle tail pages in page_cache_get_speculative()

On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> 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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