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=-5.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,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 408EBC4363D for ; Mon, 5 Oct 2020 14:12:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F2E6A205F4 for ; Mon, 5 Oct 2020 14:12:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="CiWZbP5P" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726138AbgJEOMk (ORCPT ); Mon, 5 Oct 2020 10:12:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36794 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725932AbgJEOMk (ORCPT ); Mon, 5 Oct 2020 10:12:40 -0400 Received: from mail-lf1-x141.google.com (mail-lf1-x141.google.com [IPv6:2a00:1450:4864:20::141]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 90D8DC0613CE; Mon, 5 Oct 2020 07:12:38 -0700 (PDT) Received: by mail-lf1-x141.google.com with SMTP id a9so1669896lfc.7; Mon, 05 Oct 2020 07:12:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=0As6rxUqTKqZtGA65NYtxYDkFlsoQzhzCASeNPdSS6Y=; b=CiWZbP5PMIvyDL+JAIu+09caEbIEUS87YbpK5xLAeDSoK/AQUKfI9S/UQU5D9bdkW7 1o8D5uciQIpAq4hNilWNuXlWpDB+J+jIm3Eufc6JZkMw3Qg69suKijCbx/9mLr6qtohW 5MzEVt5blas6uEuj9e2oj9daF8XkF4gdCYoY0+/0mBfb67sR90VYGJsIOTvwx2WQJ76T 9rqqUZQQmM3q8Ycfa/1NF+OCU3TkAiRBLOsAIJ0UfhyNW5IZweTsXUn63OZrzqaKt88/ QgN2DAa3mRUCwNRE0jDrF5BpkHmVw27XijYSV/LSpqKohUd8xdSv4Ff/StcyOZY8de11 IHVg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:date:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=0As6rxUqTKqZtGA65NYtxYDkFlsoQzhzCASeNPdSS6Y=; b=XjZJeKBIAMStVYzGMLFHwGEAaSSdZ4kVSo38hJ2jtfv56mfHo1udIeg67Ue5oyBP4K RNgVSztTopW+tqcW+8j8cyP6fsgSJI32UKSBnnkx5yPbJK+5MHu8VCnTBKtygJ3PmqA9 W+yOqSqmTNjk6wnMi7YiW0z7s1ge3S8TGgl+4VWEJlkLR7y3eDDeLcD6Xg76j942ukgW 0e3RUzNQdy5alVG38pyJ0RiBcJ1sGKgYc8Q0lfJ6Q2/9AHo5To3lyzRM94t1qLVuTsDI fCq0fRpO9+fxhbx7Lg48NksMXMjV0Z6pDLGotlnVGJs5vanNyQPg8O4e8Hg0IdHBcNqG s0kg== X-Gm-Message-State: AOAM531LKHrcgnZkLqhjI97c0pKVCMAnyxEB5FR3ih/3p/67Ryu4jRaB XYlun4VzSp4+MISARS0y8cc= X-Google-Smtp-Source: ABdhPJzd1nnYfxNKHqel8hrL7WlEkVN+e9coush6WVe5Ddwf2Hl2sbUdieJp6GLYKZJwna3XbEE/YA== X-Received: by 2002:a19:c6d4:: with SMTP id w203mr6194107lff.245.1601907156944; Mon, 05 Oct 2020 07:12:36 -0700 (PDT) Received: from pc636 (h5ef52e31.seluork.dyn.perspektivbredband.net. [94.245.46.49]) by smtp.gmail.com with ESMTPSA id x10sm223859ljm.71.2020.10.05.07.12.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Oct 2020 07:12:36 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 5 Oct 2020 16:12:33 +0200 To: Mel Gorman Cc: Uladzislau Rezki , Vlastimil Babka , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , Peter Zijlstra , Michal Hocko , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20201005141233.GB17959@pc636> References: <20200918194817.48921-1-urezki@gmail.com> <20200918194817.48921-3-urezki@gmail.com> <38f42ca1-ffcd-04a6-bf11-618deffa897a@suse.cz> <20200929220742.GB8768@pc636> <795d6aea-1846-6e08-ac1b-dbff82dd7133@suse.cz> <20201001192626.GA29606@pc636> <20201002080624.GB3227@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201002080624.GB3227@techsingularity.net> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 02, 2020 at 09:06:24AM +0100, Mel Gorman wrote: > On Thu, Oct 01, 2020 at 09:26:26PM +0200, Uladzislau Rezki wrote: > > > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in > > > the allocator (different from what you posted in previous version) so that it > > > only looks at the flag after it tries to allocate from pcplist and finds out > > > it's empty. So, no inventing of new page allocator entry points or checks such > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't > > > affect existing fast paths. > > > > > OK. Now i see. Please have a look below at the patch, so we fully understand > > each other. If that is something that is close to your view or not: > > > > > > t a/include/linux/gfp.h b/include/linux/gfp.h > > index c603237e006c..7e613560a502 100644 > > --- a/include/linux/gfp.h > > +++ b/include/linux/gfp.h > > @@ -39,8 +39,9 @@ struct vm_area_struct; > > #define ___GFP_HARDWALL 0x100000u > > #define ___GFP_THISNODE 0x200000u > > #define ___GFP_ACCOUNT 0x400000u > > +#define ___GFP_NO_LOCKS 0x800000u > > #ifdef CONFIG_LOCKDEP > > -#define ___GFP_NOLOCKDEP 0x800000u > > +#define ___GFP_NOLOCKDEP 0x1000000u > > #else > > #define ___GFP_NOLOCKDEP 0 > > #endif > > @@ -215,16 +216,22 @@ struct vm_area_struct; > > * %__GFP_COMP address compound page metadata. > > * > > * %__GFP_ZERO returns a zeroed page on success. > > + * > > + * %__GFP_NO_LOCKS order-0 allocation without sleepable-locks. > > + * It obtains a page from the per-cpu-list and considered as > > + * lock-less. No other actions are performed, thus it returns > > + * NULL if per-cpu-list is empty. > > */ > > #define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN) > > #define __GFP_COMP ((__force gfp_t)___GFP_COMP) > > #define __GFP_ZERO ((__force gfp_t)___GFP_ZERO) > > +#define __GFP_NO_LOCKS ((__force gfp_t)___GFP_NO_LOCKS) > > > > I'm not a fan of the GFP flag approach simply because we've had cases > before where GFP flags were used in inappropriate contexts like > __GFP_MEMALLOC which led to a surprising amount of bugs, particularly > from out-of-tree drivers but also in-tree drivers. Of course, there > are limited GFP flags available too but at least the comment should > be as robust as possible. Maybe something like > > * %__GFP_NO_LOCKS attempts order-0 allocation without sleepable-locks. It > * attempts to obtain a page without acquiring any spinlocks. This > * should only be used in a context where the holder holds a > * raw_spin_lock that cannot be released for the allocation request. > * This may be necessary in PREEMPT_RT kernels where a > * raw_spin_lock is held which does not sleep tries to acquire a > * spin_lock that can sleep with PREEMPT_RT. This should not be > * confused with GFP_ATOMIC contexts. Like atomic allocation > * requests, there is no guarantee a page will be returned and > * the caller must be able to deal with allocation failures. > * The risk of allocation failure is higher than using GFP_ATOMIC. > > It's verbose but it would be hard to misinterpret. I think we're > going to go through a period of time before people get familiar > with PREEMPT_RT-related hazards as various comments that were > true are going to be misleading for a while. > Yep, it should be properly documented for sure. Including new GFP_NOWAIT limitations, same as GFP_ATOMIC once you mentioned. Thanks! -- Vlad Rezki