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 D0C8EC433DB for ; Tue, 23 Mar 2021 07:49:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 917E6619AE for ; Tue, 23 Mar 2021 07:49:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229622AbhCWHsf (ORCPT ); Tue, 23 Mar 2021 03:48:35 -0400 Received: from mx2.suse.de ([195.135.220.15]:42840 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229464AbhCWHsV (ORCPT ); Tue, 23 Mar 2021 03:48:21 -0400 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=1616485699; 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=BM8b0sFo80e7PXN1CbnuKE6PJHUsFqB8ezy1Qvjj0z4=; b=H8fRgPDl7snu6FtB2cTPcZNvTYoKt0MHn24yfZ1TUm5uycg6kiKzPFMr0uOVyB5Hae+UhW WXt3Hg7gV+eUtfX9JuhQFfi/R5s+3pMAufGqbO+RgWIp7naPUQ2BlTzmRHZKi+FzpgsaXQ tTZBOHNAXBCkhxEnalPTH4U/ay5mJMY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 5D1C4AC3E; Tue, 23 Mar 2021 07:48:19 +0000 (UTC) Date: Tue, 23 Mar 2021 08:48:12 +0100 From: Michal Hocko To: Mike Kravetz Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shakeel Butt , Oscar Salvador , David Hildenbrand , Muchun Song , David Rientjes , Miaohe Lin , Peter Zijlstra , Matthew Wilcox , HORIGUCHI NAOYA , "Aneesh Kumar K . V" , Waiman Long , Peter Xu , Mina Almasry , Andrew Morton Subject: Re: [RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments Message-ID: References: <20210319224209.150047-1-mike.kravetz@oracle.com> <20210319224209.150047-2-mike.kravetz@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 22-03-21 09:57:14, Mike Kravetz wrote: > On 3/22/21 6:59 AM, Michal Hocko wrote: > > On Fri 19-03-21 15:42:02, Mike Kravetz wrote: > >> The number of hugetlb pages can be adjusted by writing to the > >> sysps/proc files nr_hugepages, nr_hugepages_mempolicy or > >> nr_overcommit_hugepages. There is nothing to prevent two > >> concurrent modifications via these files. The underlying routine > >> set_max_huge_pages() makes assumptions that only one occurrence is > >> running at a time. Specifically, alloc_pool_huge_page uses a > >> hstate specific variable without any synchronization. > > > > From the above it is not really clear whether the unsynchronized nature > > of set_max_huge_pages is really a problem or a mere annoynce. I suspect > > the later because counters are properly synchronized with the > > hugetlb_lock. It would be great to clarify that. > > > > It is a problem and an annoyance. > > The problem is that alloc_pool_huge_page -> for_each_node_mask_to_alloc is > called after dropping the hugetlb lock. for_each_node_mask_to_alloc > uses the helper hstate_next_node_to_alloc which uses and modifies > h->next_nid_to_alloc. Worst case would be two instances of set_max_huge_pages > trying to allocate pages on different sets of nodes. Pages could get > allocated on the wrong nodes. Yes, what I meant by the annoyance. On the other hand a parallel access to a global knob mantaining a global resource should be expected to have some side effects without an external synchronization unless it is explicitly documented that such an access is synchronized internally. > I really doubt this problem has ever been experienced in practice. > However, when looking at the code in was a real annoyance. :) IMHO it would be a bit of a stretch to consider it a real life problem. > I'll update the commit message to be more clear. Thanks! Clarification will definitely help. -- Michal Hocko SUSE Labs