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=-7.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,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 222F4C433DB for ; Wed, 27 Jan 2021 10:41:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CF987205F4 for ; Wed, 27 Jan 2021 10:41:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236020AbhA0KlS (ORCPT ); Wed, 27 Jan 2021 05:41:18 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:39803 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235993AbhA0KiA (ORCPT ); Wed, 27 Jan 2021 05:38:00 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611743793; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SCPzZ58zrnooW9g+/emDghb7lm/E/AgOMGI9bAy3zYE=; b=e5GJqwMa9axlJcsX5B1CCDT5Q4Q5YFdeXL9vBLMBTDgpx5oHIIKxd7RZsOn5z4qcIuqKad Xcr/RUCBIlAhhGLBWsMgRUsLd+/dEABCACPio6kO5T0QdQPsgRDuSNgLWax+NndopR4p7j py/Hf4buG8m6l5AU47tjRU4Ime/lHQQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-71-lIVf9Fe2Nnu587VehfUqmw-1; Wed, 27 Jan 2021 05:36:29 -0500 X-MC-Unique: lIVf9Fe2Nnu587VehfUqmw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 769BE190B2A2; Wed, 27 Jan 2021 10:36:25 +0000 (UTC) Received: from [10.36.114.237] (ovpn-114-237.ams2.redhat.com [10.36.114.237]) by smtp.corp.redhat.com (Postfix) with ESMTP id 504195C5FD; Wed, 27 Jan 2021 10:36:16 +0000 (UTC) From: David Hildenbrand To: Oscar Salvador Cc: Muchun Song , corbet@lwn.net, mike.kravetz@oracle.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, viro@zeniv.linux.org.uk, akpm@linux-foundation.org, paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, rdunlap@infradead.org, oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, almasrymina@google.com, rientjes@google.com, willy@infradead.org, mhocko@suse.com, song.bao.hua@hisilicon.com, naoya.horiguchi@nec.com, duanxiongchun@bytedance.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org References: <20210117151053.24600-1-songmuchun@bytedance.com> <20210117151053.24600-6-songmuchun@bytedance.com> <20210126092942.GA10602@linux> <6fe52a7e-ebd8-f5ce-1fcd-5ed6896d3797@redhat.com> <20210126145819.GB16870@linux> <259b9669-0515-01a2-d714-617011f87194@redhat.com> <20210126153448.GA17455@linux> <9475b139-1b33-76c7-ef5c-d43d2ea1dba5@redhat.com> Organization: Red Hat GmbH Subject: Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages associated with each HugeTLB page Message-ID: Date: Wed, 27 Jan 2021 11:36:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <9475b139-1b33-76c7-ef5c-d43d2ea1dba5@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26.01.21 16:56, David Hildenbrand wrote: > On 26.01.21 16:34, Oscar Salvador wrote: >> On Tue, Jan 26, 2021 at 04:10:53PM +0100, David Hildenbrand wrote: >>> The real issue seems to be discarding the vmemmap on any memory that has >>> movability constraints - CMA and ZONE_MOVABLE; otherwise, as discussed, we >>> can reuse parts of the thingy we're freeing for the vmemmap. Not that it >>> would be ideal: that once-a-huge-page thing will never ever be a huge page >>> again - but if it helps with OOM in corner cases, sure. >> >> Yes, that is one way, but I am not sure how hard would it be to implement. >> Plus the fact that as you pointed out, once that memory is used for vmemmap >> array, we cannot use it again. >> Actually, we would fragment the memory eventually? >> >>> Possible simplification: don't perform the optimization for now with free >>> huge pages residing on ZONE_MOVABLE or CMA. Certainly not perfect: what >>> happens when migrating a huge page from ZONE_NORMAL to (ZONE_MOVABLE|CMA)? >> >> But if we do not allow theose pages to be in ZONE_MOVABLE or CMA, there is no >> point in migrate them, right? > > Well, memory unplug "could" still work and migrate them and > alloc_contig_range() "could in the future" still want to migrate them > (virtio-mem, gigantic pages, powernv memtrace). Especially, the latter > two don't work with ZONE_MOVABLE/CMA. But, I mean, it would be fair > enough to say "there are no guarantees for > alloc_contig_range()/offline_pages() with ZONE_NORMAL, so we can break > these use cases when a magic switch is flipped and make these pages > non-migratable anymore". > > I assume compaction doesn't care about huge pages either way, not sure > about numa balancing etc. > > > However, note that there is a fundamental issue with any approach that > allocates a significant amount of unmovable memory for user-space > purposes (excluding CMA allocations for unmovable stuff, CMA is > special): pairing it with ZONE_MOVABLE becomes very tricky as your user > space might just end up eating all kernel memory, although the system > still looks like there is plenty of free memory residing in > ZONE_MOVABLE. I mentioned that in the context of secretmem in a reduced > form as well. > > We theoretically have that issue with dynamic allocation of gigantic > pages, but it's something a user explicitly/rarely triggers and it can > be documented to cause problems well enough. We'll have the same issue > with GUP+ZONE_MOVABLE that Pavel is fixing right now - but GUP is > already known to be broken in various ways and that it has to be treated > in a special way. I'd like to limit the nasty corner cases. > > Of course, we could have smart rules like "don't online memory to > ZONE_MOVABLE automatically when the magic switch is active". That's just > ugly, but could work. > Extending on that, I just discovered that only x86-64, ppc64, and arm64 really support hugepage migration. Maybe one approach with the "magic switch" really would be to disable hugepage migration completely in hugepage_migration_supported(), and consequently making hugepage_movable_supported() always return false. Huge pages would never get placed onto ZONE_MOVABLE/CMA and cannot be migrated. The problem I describe would apply (careful with using ZONE_MOVABLE), but well, it can at least be documented. -- Thanks, David / dhildenb