From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965203AbbLOMDX (ORCPT ); Tue, 15 Dec 2015 07:03:23 -0500 Received: from mail-wm0-f47.google.com ([74.125.82.47]:37815 "EHLO mail-wm0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964921AbbLOMDV (ORCPT ); Tue, 15 Dec 2015 07:03:21 -0500 Date: Tue, 15 Dec 2015 14:03:18 +0200 From: "Kirill A. Shutemov" To: Michal Hocko Cc: Andrew Morton , Minchan Kim , linux-mm@kvack.org, LKML Subject: Re: isolate_lru_page on !head pages Message-ID: <20151215120318.GA11497@node.shutemov.name> References: <20151209130204.GD30907@dhcp22.suse.cz> <20151214120456.GA4201@node.shutemov.name> <20151215085232.GB14350@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151215085232.GB14350@dhcp22.suse.cz> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 15, 2015 at 09:52:33AM +0100, Michal Hocko wrote: > On Mon 14-12-15 14:04:56, Kirill A. Shutemov wrote: > > On Wed, Dec 09, 2015 at 02:02:05PM +0100, Michal Hocko wrote: > > > Hi Kirill, > > > > [ sorry for late reply, just back from vacation. ] > > > > > while looking at the issue reported by Minchan [1] I have noticed that > > > there is nothing to prevent from "isolating" a tail page from LRU because > > > isolate_lru_page checks PageLRU which is > > > PAGEFLAG(LRU, lru, PF_HEAD) > > > so it is checked on the head page rather than the given page directly > > > but the rest of the operation is done on the given (tail) page. > > > > Looks like most (all?) callers already exclude PTE-mapped THP already one > > way or another. > > I can see e.g. do_move_page_to_node_array not doing a similar thing. It > isolates and then migrates potentially a tail page. No, it doesn't. follow_page(FOLL_SPLIT) would split THP pages. > I haven't looked closer whether there is other hand break on the way > though. The point I was trying to make is that this is really _subtle_. > We are changing something else than we operate later on. > > > Probably, VM_BUG_ON_PAGE(PageTail(page), page) in isolate_lru_page() would > > be appropriate. > > > > > This is really subtle because this expects that every caller of this > > > function checks for the tail page otherwise we would clobber statistics > > > and who knows what else (I haven't checked that in detail) as the page > > > cannot be on the LRU list and the operation makes sense only on the head > > > page. > > > > > > Would it make more sense to make PageLRU PF_ANY? That would return > > > false for PageLRU on any tail page and so it would be ignored by > > > isolate_lru_page. > > > > I don't think this is right way to go. What we put on LRU is compound > > page, not 4k subpages. PageLRU() should return true if the compound page > > is on LRU regardless if you ask for head or tail page. > > Hmm, but then we should operate on the head page because that is what > PageLRU operated on, no? head page is what linked into LRU, but not nessesary the way we obtain the page to check. If we check PageLRU(pte_page(*pte)) it should produce the right result. > > False-negatives PageLRU() can be as bad as bug Minchan reported, but > > perhaps more silent. > > -- > Michal Hocko > SUSE Labs -- Kirill A. Shutemov