From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx160.postini.com [74.125.245.160]) by kanga.kvack.org (Postfix) with SMTP id B32576B0007 for ; Sun, 27 Jan 2013 17:07:59 -0500 (EST) Received: by mail-da0-f46.google.com with SMTP id p5so939652dak.5 for ; Sun, 27 Jan 2013 14:07:58 -0800 (PST) Date: Sun, 27 Jan 2013 14:08:00 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH 5/11] ksm: get_ksm_page locked In-Reply-To: <1359254187.4159.10.camel@kernel> Message-ID: References: <1359254187.4159.10.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 Sat, 26 Jan 2013, Simon Jeons wrote: > On Fri, 2013-01-25 at 18:00 -0800, Hugh Dickins wrote: > > In some places where get_ksm_page() is used, we need the page to be locked. > > > > In function get_ksm_page, why check page->mapping => > get_page_unless_zero => check page->mapping instead of > get_page_unless_zero => check page->mapping, because > get_page_unless_zero is expensive? Yes, it's more expensive. > > > When KSM migration is fully enabled, we shall want that to make sure that > > the page just acquired cannot be migrated beneath us (raised page count is > > only effective when there is serialization to make sure migration notices). > > Whereas when navigating through the stable tree, we certainly do not want > > What's the meaning of "navigating through the stable tree"? Finding the right place in the stable tree, as stable_tree_search() and stable_tree_insert() do. > > > to lock each node (raised page count is enough to guarantee the memcmps, > > even if page is migrated to another node). > > > > Since we're about to add another use case, add the locked argument to > > get_ksm_page() now. > > Why the parameter lock passed from stable_tree_search/insert is true, > but remove_rmap_item_from_tree is false? The other way round? remove_rmap_item_from_tree needs the page locked, because it's about to modify the list: that's secured (e.g. against concurrent KSM page reclaim) by the page lock. stable_tree_search and stable_tree_insert do not need intermediate nodes to be locked: get_page is enough to secure the page contents for memcmp, and we don't want a pointless wait for exclusive page lock on every intermediate node. -- 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