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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 3A639C41517 for ; Wed, 24 Jul 2019 18:59:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1022221951 for ; Wed, 24 Jul 2019 18:59:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563994757; bh=z75XAm2dZpjSYG5A9x4KxqUsbd6EWzTFxBbcWmCtKvs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=kovghY91kQ+PFin3MrRE4c1OoT7niibEo+0C44S9r9DwCxK/7UUt4JjJAKJg1/Cc3 Q5vM90nsAzMQ1uTT0v+AAf0EUjp4AbDF63AWV5NgpgRflKDvCgWFvwmEh31WXKhNfE DFxaq/yhSqfCdbOI+NEWnairRyP0O4/Ysl/XKUcE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728793AbfGXS7P (ORCPT ); Wed, 24 Jul 2019 14:59:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:46982 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725883AbfGXS7P (ORCPT ); Wed, 24 Jul 2019 14:59:15 -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 170CCADC1; Wed, 24 Jul 2019 18:59:14 +0000 (UTC) Date: Wed, 24 Jul 2019 20:59:10 +0200 From: Michal Hocko To: Jason Gunthorpe Cc: Christoph Hellwig , Ralph Campbell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, =?iso-8859-1?B?Suly9G1l?= Glisse , Ben Skeggs Subject: Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier_range Message-ID: <20190724185910.GF6410@dhcp22.suse.cz> References: <20190723210506.25127-1-rcampbell@nvidia.com> <20190724070553.GA2523@lst.de> <20190724152858.GB28493@ziepe.ca> <20190724175858.GC6410@dhcp22.suse.cz> <20190724180837.GF28493@ziepe.ca> <20190724185617.GE6410@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190724185617.GE6410@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 Wed 24-07-19 20:56:17, Michal Hocko wrote: > On Wed 24-07-19 15:08:37, Jason Gunthorpe wrote: > > On Wed, Jul 24, 2019 at 07:58:58PM +0200, Michal Hocko wrote: > [...] > > > Maybe new users have started relying on a new semantic in the meantime, > > > back then, none of the notifier has even started any action in blocking > > > mode on a EAGAIN bailout. Most of them simply did trylock early in the > > > process and bailed out so there was nothing to do for the range_end > > > callback. > > > > Single notifiers are not the problem. I tried to make this clear in > > the commit message, but lets be more explicit. > > > > We have *two* notifiers registered to the mm, A and B: > > > > A invalidate_range_start: (has no blocking) > > spin_lock() > > counter++ > > spin_unlock() > > > > A invalidate_range_end: > > spin_lock() > > counter-- > > spin_unlock() > > > > And this one: > > > > B invalidate_range_start: (has blocking) > > if (!try_mutex_lock()) > > return -EAGAIN; > > counter++ > > mutex_unlock() > > > > B invalidate_range_end: > > spin_lock() > > counter-- > > spin_unlock() > > > > So now the oom path does: > > > > invalidate_range_start_non_blocking: > > for each mn: > > a->invalidate_range_start > > b->invalidate_range_start > > rc = EAGAIN > > > > Now we SKIP A's invalidate_range_end even though A had no idea this > > would happen has state that needs to be unwound. A is broken. > > > > B survived just fine. > > > > A and B *alone* work fine, combined they fail. > > But that requires that they share some state, right? > > > When the commit was landed you can use KVM as an example of A and RDMA > > ODP as an example of B > > Could you point me where those two share the state please? KVM seems to > be using kvm->mmu_notifier_count but I do not know where to look for the > RDMA... Scratch that. ELONGDAY... I can see your point. It is all or nothing that doesn't really work here. Looking back at your patch it seems reasonable but I am not sure what is supposed to be a behavior for notifiers that failed. -- Michal Hocko SUSE Labs