From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx199.postini.com [74.125.245.199]) by kanga.kvack.org (Postfix) with SMTP id 9C45B6B0062 for ; Tue, 21 May 2013 16:49:54 -0400 (EDT) Message-ID: <519BDDEF.9020705@sr71.net> Date: Tue, 21 May 2013 13:49:51 -0700 From: Dave Hansen MIME-Version: 1.0 Subject: Re: [PATCHv4 17/39] thp, mm: handle tail pages in page_cache_get_speculative() References: <1368321816-17719-1-git-send-email-kirill.shutemov@linux.intel.com> <1368321816-17719-18-git-send-email-kirill.shutemov@linux.intel.com> In-Reply-To: <1368321816-17719-18-git-send-email-kirill.shutemov@linux.intel.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrea Arcangeli , Andrew Morton , Al Viro , Hugh Dickins , Wu Fengguang , Jan Kara , Mel Gorman , linux-mm@kvack.org, Andi Kleen , Matthew Wilcox , "Kirill A. Shutemov" , Hillf Danton , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote: > From: "Kirill A. Shutemov" > > 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org