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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 36FBDCA9EAF for ; Mon, 21 Oct 2019 13:48:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 145B52053B for ; Mon, 21 Oct 2019 13:48:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729109AbfJUNsx (ORCPT ); Mon, 21 Oct 2019 09:48:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:60766 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728083AbfJUNsw (ORCPT ); Mon, 21 Oct 2019 09:48:52 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 22766B730; Mon, 21 Oct 2019 13:48:50 +0000 (UTC) Date: Mon, 21 Oct 2019 15:48:48 +0200 From: Oscar Salvador To: Michal Hocko Cc: n-horiguchi@ah.jp.nec.com, mike.kravetz@oracle.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 11/16] mm,hwpoison: Rework soft offline for in-use pages Message-ID: <20191021134846.GB11330@linux> References: <20191017142123.24245-1-osalvador@suse.de> <20191017142123.24245-12-osalvador@suse.de> <20191018123901.GN5017@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191018123901.GN5017@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 18, 2019 at 02:39:01PM +0200, Michal Hocko wrote: > > I am sorry but I got lost in the above description and I cannot really > make much sense from the code either. Let me try to outline the way how > I think about this. > > Say we have a pfn to hwpoison. We have effectivelly three possibilities > - page is poisoned already - done nothing to do > - page is managed by the buddy allocator - excavate from there > - page is in use > > The last category is the most interesting one. There are essentially > three classes of pages > - freeable > - migrateable > - others > > We cannot do really much about the last one, right? Do we mark them > HWPoison anyway? We can only perform actions on LRU/Movable pages or hugetlb pages. So unless the page does not fall into those areas, we do not do anything with them. > Freeable should be simply marked HWPoison and freed. > For all those migrateable, we simply do migrate and mark HWPoison. > Now the main question is how to handle HWPoison page when it is freed > - aka last reference is dropped. The main question is whether the last > reference is ever dropped. If yes then the free_pages_prepare should > never release it to the allocator (some compound destructors would have > to special case as well, e.g. hugetlb would have to hand over to the > allocator rather than a pool). If not then the page would be lingering > potentially with some state bound to it (e.g. memcg charge). So I > suspect you want the former. For non-hugetlb pages, we do not call put_page in the migration path, but we do it in page_handle_poison, after the page has been flagged as hwpoison. Then the check in free_papes_prepare will see that the page is hwpoison and will bail out, so the page is not released into the allocator/pcp lists. Hugetlb pages follow a different methodology. They are dissolved, and then we split the higher-order page and take the page off the buddy. The problem is that while it is easy to hold a non-hugetlb page, doing the same for hugetlb pages is not that easy: 1) we would need to hook in enqueue_hugetlb_page so the page is not enqueued into hugetlb freelists 2) when trying to free a hugetlb page, we would need to do as we do for gigantic pages now, and that is breaking down the pages into order-0 pages and release them to the buddy (so the check in free_papges_prepare would skip the hwpoison page). Trying to handle a higher-order hwpoison page in free_pages_prepare is a bit complicated. There is one thing I was unsure though. Bailing out at the beginning of free_pages_prepare if the page is hwpoison means that the calls to - __memcg_kmem_uncharge - page_cpupid_reset_last - reset_page_owner - ... will not be performed. I thought this is right because the page is not really "free", it is just unusable, so.. it should be still charged to the memcg? -- Oscar Salvador SUSE L3