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 53B126B0009 for ; Sun, 27 Jan 2013 22:44:55 -0500 (EST) Received: by mail-da0-f41.google.com with SMTP id e20so1040416dak.28 for ; Sun, 27 Jan 2013 19:44:54 -0800 (PST) Date: Sun, 27 Jan 2013 19:44:56 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH 7/11] ksm: make KSM page migration possible In-Reply-To: <1359333683.6763.13.camel@kernel> Message-ID: References: <1359265635.6763.0.camel@kernel> <1359333683.6763.13.camel@kernel> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Simon Jeons Cc: Andrew Morton , Petr Holasek , Andrea Arcangeli , Izik Eidus , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Sun, 27 Jan 2013, Simon Jeons wrote: > On Sun, 2013-01-27 at 15:12 -0800, Hugh Dickins wrote: > > On Sat, 26 Jan 2013, Simon Jeons wrote: > > > > > > Could you explain why need check page->mapping twice after get page? > > > > Once for the !locked case, which should not return page if mapping changed. > > Once for the locked case, which should not return page if mapping changed. > > We could use "else", but that wouldn't be an improvement. > > But for locked case, page->mapping will be check twice. Thrice. I'm beginning to wonder: you do realize that page->mapping is volatile, from the point of view of get_ksm_page()? That is the whole point of why get_ksm_page() exists. I can see that the word "volatile" is not obviously used here - it's tucked away inside the ACCESS_ONCE() - but I thought the descriptions of races and barriers made that obvious. If the comments here haven't helped enough, please take a look at git commit 4035c07a8959 "ksm: take keyhole reference to page". -- 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