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 8A642C433E0 for ; Thu, 11 Mar 2021 09:50:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 39B8964D74 for ; Thu, 11 Mar 2021 09:50:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231991AbhCKJtn (ORCPT ); Thu, 11 Mar 2021 04:49:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55628 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232123AbhCKJti (ORCPT ); Thu, 11 Mar 2021 04:49:38 -0500 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 62A10C061574 for ; Thu, 11 Mar 2021 01:49:38 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=qTTXD4rDBXLQmKbDmwsmX4CcXJeQvWfH6qNaQAGLaWQ=; b=dQPNckSHiEl3oEVreFjBKR+61Y M3JWAzF9I9MiI+7YJaHHBZpjiIgk/ky4xehaAWQwDIa+imXla9ugmp8s9x2CsFHjxrIKMPGO2MzaK 5+mAbrFn1H59FIB6OuuXvnHO3TktcR2vDMj+yvoBj+2BbRYD2rQntsivq01hjAKkubPvO0Gbw0BOZ TL9UdTtIHRTfSoZk5i3a8h8aeFU25gy0Y/A7wsRnQCT+22fQVRNjDs4tUvlAV8kWQXRNmvwn5p8ds QVPOFfoiKV5ZqZFTRAzcHNbK6br3+OwBTy9agVa6isH+z/aAFxODpOgXkU2sX9LgbPkHazFurDZUw t0lQMBaA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lKHwb-008mST-HR; Thu, 11 Mar 2021 09:49:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id DE17A300455; Thu, 11 Mar 2021 10:49:23 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D06EE2027A4AC; Thu, 11 Mar 2021 10:49:23 +0100 (CET) Date: Thu, 11 Mar 2021 10:49:23 +0100 From: Peter Zijlstra To: Mike Kravetz Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Michal Hocko , "Paul E . McKenney" , Shakeel Butt , tglx@linutronix.de, john.ogness@linutronix.de, urezki@gmail.com, ast@fb.com, Eric Dumazet , Mina Almasry , Andrew Morton Subject: Re: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for in_atomic use Message-ID: References: <20210311021321.127500-1-mike.kravetz@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210311021321.127500-1-mike.kravetz@oracle.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 10, 2021 at 06:13:21PM -0800, Mike Kravetz wrote: > put_page does not correctly handle all calling contexts for hugetlb > pages. This was recently discussed in the threads [1] and [2]. > > free_huge_page is the routine called for the final put_page of huegtlb > pages. Since at least the beginning of git history, free_huge_page has > acquired the hugetlb_lock to move the page to a free list and possibly > perform other processing. When this code was originally written, the > hugetlb_lock should have been made irq safe. > > For many years, nobody noticed this situation until lockdep code caught > free_huge_page being called from irq context. By this time, another > lock (hugetlb subpool) was also taken in the free_huge_page path. AFAICT there's no actual problem with making spool->lock IRQ-safe too. > In addition, hugetlb cgroup code had been added which could hold > hugetlb_lock for a considerable period of time. cgroups, always bloody cgroups. The scheduler (and a fair number of other places) get to deal with cgroups with IRQs disabled, so I'm sure this can too. > Because of this, commit > c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task > context") was added to address the issue of free_huge_page being called > from irq context. That commit hands off free_huge_page processing to a > workqueue if !in_task. > > The !in_task check handles the case of being called from irq context. > However, it does not take into account the case when called with irqs > disabled as in [1]. > > To complicate matters, functionality has been added to hugetlb > such that free_huge_page may block/sleep in certain situations. The > hugetlb_lock is of course dropped before potentially blocking. AFAICT that's because CMA, right? That's only hstate_is_gigantic() and free_gigantic_page() that has that particular trainwreck. So you could move the workqueue there, and leave all the other hugetlb sizes unaffected. Afaict if you limit the workqueue crud to cma_clear_bitmap(), you don't get your.. > One way to handle all calling contexts is to have free_huge_page always > send pages to the workqueue for processing. This idea was briefly > discussed here [3], but has some undesirable side effects. ... user visible side effects either. > Ideally, the hugetlb_lock should have been irq safe from the beginning > and any code added to the free_huge_page path should have taken this > into account. However, this has not happened. The code today does have > the ability to hand off requests to a workqueue. It does this for calls > from irq context. Changing the check in the code from !in_task to > in_atomic would handle the situations when called with irqs disabled. > However, it does not not handle the case when called with a spinlock > held. This is needed because the code could block/sleep. I'll argue the current workqueue thing is in the wrong place to begin with. So how about you make hugetlb_lock and spool->lock IRQ-safe, move thw workqueue thingy into cma_release(), and then worry about optimizing the cgroup crap? Correctness first, performance second. Also, if you really care about performance, not using cgroups is a very good option anyway.