From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D6622C433E0 for ; Mon, 15 Feb 2021 09:01:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 85F4A64E05 for ; Mon, 15 Feb 2021 09:01:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230005AbhBOJBa (ORCPT ); Mon, 15 Feb 2021 04:01:30 -0500 Received: from mx2.suse.de ([195.135.220.15]:50006 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229783AbhBOJBY (ORCPT ); Mon, 15 Feb 2021 04:01:24 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613379637; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ZQWJ8QEjpLvqVVRPY5L/a3kejC8e54tG2MpJnL0Ffzc=; b=SzyiVD+4a++QK/wfXH2nED0hIs8vUHBhLAtgznr4BTzH0zaEV6BzZC+8MgJNWMnaw4QDLy tlNHAFzPgQ7ii+qV0vpKS/pRjU5ZFmHvzYZKRvhy4uFBd7zfJnG4yqbpoXzpxX4L0+RExX NlcHpldaXyaRwVp4zdvixsz2TG8fFUU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3B38DAE76; Mon, 15 Feb 2021 09:00:37 +0000 (UTC) Date: Mon, 15 Feb 2021 10:00:31 +0100 From: Michal Hocko To: Mike Rapoport , Mel Gorman Cc: David Hildenbrand , Andrew Morton , Andrea Arcangeli , Baoquan He , Borislav Petkov , Chris Wilson , "H. Peter Anvin" , Ingo Molnar , Linus Torvalds , =?utf-8?Q?=C5=81ukasz?= Majczak , Mike Rapoport , Qian Cai , "Sarvela, Tomi P" , Thomas Gleixner , Vlastimil Babka , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout Message-ID: References: <20210208110820.6269-1-rppt@kernel.org> <20210214180016.GO242749@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210214180016.GO242749@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 14-02-21 20:00:16, Mike Rapoport wrote: > On Fri, Feb 12, 2021 at 02:18:20PM +0100, Michal Hocko wrote: > > On Fri 12-02-21 11:42:15, David Hildenbrand wrote: > > > On 12.02.21 11:33, Michal Hocko wrote: > > [...] > > > > I have to digest this but my first impression is that this is more heavy > > > > weight than it needs to. Pfn walkers should normally obey node range at > > > > least. The first pfn is usually excluded but I haven't seen real > > > > > > We've seen examples where this is not sufficient. Simple example: > > > > > > Have your physical memory end within a memory section. Easy via QEMU, just > > > do a "-m 4000M". The remaining part of the last section has fake/wrong > > > node/zone info. > > > > Does this really matter though. If those pages are reserved then nobody > > will touch them regardless of their node/zone ids. > > > > > Hotplug memory. The node/zone gets resized such that PFN walkers might > > > stumble over it. > > > > > > The basic idea is to make sure that any initialized/"online" pfn belongs to > > > exactly one node/zone and that the node/zone spans that PFN. > > > > Yeah, this sounds like a good idea but what is the poper node for hole > > between two ranges associated with a different nodes/zones? This will > > always be a random number. We should have a clear way to tell "do not > > touch those pages" and PageReserved sounds like a good way to tell that. > > Nobody should touch reserved pages, but I don't think we can ensure that. Touching a reserved page which doesn't belong to you is a bug. Sure we cannot enforce that rule by runtime checks. But incorrect/misleading zone/node association is the least of the problem when somebody already does that. > We can correctly set the zone links for the reserved pages for holes in the > middle of a zone based on the architecture constraints and with only the > holes in the beginning/end of the memory will be not spanned by any > node/zone which in practice does not seem to be a problem as the VM_BUG_ON > in set_pfnblock_flags_mask() never triggered on pfn 0. I really fail to see what you mean by correct zone/node for a memory range which is not associated with any real node. > I believe that any improvement in memory map consistency is a step forward. I do agree but we are talking about a subtle bug (VM_BUG_ON) which would be better of with a simplistic fix first. You can work on consistency improvements on top of that. > > > > problems with that. The VM_BUG_ON blowing up is really bad but as said > > > > above we can simply make it less offensive in presence of reserved pages > > > > as those shouldn't reach that path AFAICS normally. > > > > > > Andrea tried tried working around if via PG_reserved pages and it resulted > > > in quite some ugly code. Andrea also noted that we cannot rely on any random > > > page walker to do the right think when it comes to messed up node/zone info. > > > > I am sorry, I haven't followed previous discussions. Has the removal of > > the VM_BUG_ON been considered as an immediate workaround? > > It was never discussed, but I'm not sure it's a good idea. > > Judging by the commit message that introduced the VM_BUG_ON (commit > 86051ca5eaf5 ("mm: fix usemap initialization")) there was yet another > inconsistency in the memory map that required a special care. Can we actually explore that path before adding yet additional complexity and potentially a very involved fix for a subtle problem? Mel who is author of this code might help us out. I have to say I do not see the point for the VM_BUG_ON other than a better debuggability. If there is a real incosistency problem as a result then we should be handling that situation for non debugging kernels as well. -- Michal Hocko SUSE Labs