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=-3.8 required=3.0 tests=BAYES_00, 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 CD890C433E0 for ; Wed, 10 Feb 2021 14:24:29 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 698F464E7C for ; Wed, 10 Feb 2021 14:24:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 698F464E7C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id EBBE16B006C; Wed, 10 Feb 2021 09:24:28 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E6CFD6B006E; Wed, 10 Feb 2021 09:24:28 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D822B6B0071; Wed, 10 Feb 2021 09:24:28 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0222.hostedemail.com [216.40.44.222]) by kanga.kvack.org (Postfix) with ESMTP id BE83F6B006C for ; Wed, 10 Feb 2021 09:24:28 -0500 (EST) Received: from smtpin28.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 89A11184D8FB3 for ; Wed, 10 Feb 2021 14:24:28 +0000 (UTC) X-FDA: 77802578616.28.DCB9496 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf26.hostedemail.com (Postfix) with ESMTP id B4FA5407F8CD for ; Wed, 10 Feb 2021 14:24:26 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 8297BB152; Wed, 10 Feb 2021 14:24:26 +0000 (UTC) Date: Wed, 10 Feb 2021 15:24:24 +0100 From: Oscar Salvador To: David Hildenbrand Cc: Mike Kravetz , Muchun Song , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/2] mm,page_alloc: Make alloc_contig_range handle free hugetlb pages Message-ID: <20210210142424.GC3636@localhost.localdomain> References: <20210208103812.32056-1-osalvador@suse.de> <20210208103812.32056-3-osalvador@suse.de> <9ed946df-9c6c-9a4d-4be9-2f32809974f7@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9ed946df-9c6c-9a4d-4be9-2f32809974f7@redhat.com> X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B4FA5407F8CD X-Stat-Signature: 1kjt84itmnhc5u1hz4hzdp69pymjrfff Received-SPF: none (suse.de>: No applicable sender policy available) receiver=imf26; identity=mailfrom; envelope-from=""; helo=mx2.suse.de; client-ip=195.135.220.15 X-HE-DKIM-Result: none/none X-HE-Tag: 1612967066-836165 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Feb 10, 2021 at 09:23:59AM +0100, David Hildenbrand wrote: > On 08.02.21 11:38, Oscar Salvador wrote: > > Free hugetlb pages are trickier to handle as to in order to guarantee > > no userspace appplication disruption, we need to replace the > > current free hugepage with a new one. > > > > In order to do that, a new function called alloc_and_dissolve_huge_page > > in introduced. > > This function will first try to get a new fresh hugetlb page, and if it > > succeeds, it will dissolve the old one. > > > > Thanks for looking into this! Can we move this patch to #1 in the series? It > is the easier case. > > I also wonder if we should at least try on the memory unplug path to keep > nr_pages by at least trying to allocate at new one if required, and printing > a warning if that fails (after all, we're messing with something configured > by the admin - "nr_pages"). Note that gigantic pages are special (below). So, do you mean to allocate a new fresh hugepage in case we have a free hugetlb page within the range we are trying to offline? That makes some sense I guess. I can have a look at that, and make hotplug code use the new alloc_and_dissolve(). Thanks for bringing this up, it is somsething I did not think about. > > + /* > > + * Free hugetlb page. Allocate a new one and > > + * dissolve this is if succeed. > > + */ > > + if (alloc_and_dissolve_huge_page(page)) { > > + unsigned long order = buddy_order_unsafe(page); > > + > > + low_pfn += (1UL << order) - 1; > > + continue; > > + } > > > > Note that there is a very ugly corner case we will have to handle gracefully > (I think also in patch #1): > > Assume you allocated a gigantic page (and assume that we are not using CMA > for gigantic pages for simplicity). Assume you want to allocate another one. > alloc_pool_huge_page()->...->alloc_contig_pages() will stumble over the > first allocated page. It will try to alloc_and_dissolve_huge_page() the > existing gigantic page. To do that, it will > alloc_pool_huge_page()->...->alloc_contig_pages() ... and so on. Bad. Heh, I was too naive. I have to confess I completely forgot about gigantic pages and this cyclic dependency. > We really don't want to mess with gigantic pages (migrate, dissolve) while > allocating a gigantic page. I think the easiest (and cleanest) way forward > is to not mess (isolate, migrate, dissolve) with gigantic pages at all. > > Gigantic pages are not movable, so they won't be placed on random CMA / > ZONE_MOVABLE. > > Some hstate_is_gigantic(h) calls (maybe inside > alloc_and_dissolve_huge_page() ? ) along with a nice comment might be good > enough to avoid having to pass down some kind of alloc_contig context. I > even think that should be handled inside > > (the main issue is that in contrast to CMA, plain alloc_contig_pages() has > no memory about which parts were allocated and will simply try re-allocating > what it previously allocated and never freed - which is usually fine, unless > we're dealing with such special cases) > > Apart from that, not messing with gigantic pages feels like the right > approach (allocating/migrating gigantic pages is just horribly slow and most > probably not worth it anyway). Yes, I also agree that we should leave out gigantic pages, at least for now. We might make it work in the future but I cannot come up with a fancy way to work around that right now, so it makes sense to cut down the complexity here. Thanks David for the insight! -- Oscar Salvador SUSE L3