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 57B25C4363C for ; Wed, 7 Oct 2020 11:02:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E4FCA20872 for ; Wed, 7 Oct 2020 11:02:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="L7Xj6RZJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728351AbgJGLC4 (ORCPT ); Wed, 7 Oct 2020 07:02:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56142 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726219AbgJGLCv (ORCPT ); Wed, 7 Oct 2020 07:02:51 -0400 Received: from mail-lj1-x243.google.com (mail-lj1-x243.google.com [IPv6:2a00:1450:4864:20::243]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 07288C061755; Wed, 7 Oct 2020 04:02:50 -0700 (PDT) Received: by mail-lj1-x243.google.com with SMTP id a23so737089ljp.5; Wed, 07 Oct 2020 04:02:50 -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=CbjONhvllNSyW702K8bZjURMem2CyW8UYU/jLhqWZTM=; b=L7Xj6RZJ7dh8W0ZgTwJU6Go5ids6sTbDmO5IN+gOOeP6Uo6dtXAFmpVFdEmHpOHy2x +gsylc2e4ZjS0grEqCwE2GOkMKlXg8MWbUHrAwjm2/IwJmqQX5Gfy9Ib9GyIo9/VRBiG P4eKkh4ELpYGD5B0Xuy3kgvIW2SdgCYIwHmHEb1ba2hVqy1aA2YHRBmDlOQybAQcL2qX NClVuojhQ9jtB3da3+98+TsL+xgJmFMF/zQSIwgi2OPCLwHYrPLCwug7Du53qguNwlxR PznpTCphf4twLxgDtmOeB51DmVLtlRHyHCAKjjfHxQd/HvSeKEdw1HNWF020nbOdw/Z3 Tv6w== 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=CbjONhvllNSyW702K8bZjURMem2CyW8UYU/jLhqWZTM=; b=Jd6YXd81Rq+F1XAqxju5cdlr38w/aza2N9sSUefTN6GOW4+4koHQCctSjSJcwZkPNY dQFzMFeqsuii/3u+g0rSFw1hzBEgkieb7I/BbUKsgPNm2veUEqccezw55HeWX5ltwxOB /E0nSmbcXXkDmKMivx8WMz53PipyuF6t8w2GErkCq6fED6eAN7OnK/yBhW2XzvH1N3yo KC9MUpTe+VZry6wZxjP729L/lWxvp9hHYxTq7rlyXdC2NTFEo1CjudnOeh2vK4T86YAS m+eS1f/UwfQ6ynbiuY1wWHc9492Dfvna5DIfb2NsaniNxGpNe3M59xTJEuBuOdmz/hw0 FmpA== X-Gm-Message-State: AOAM533mnYlOagTQ2MlbjuO6HddvFeyexbqNVJS8SNWfjNDPL4pHZ117 pq6Fn7AgEdiTcPNT6Ifmex8= X-Google-Smtp-Source: ABdhPJw4UGU50WDP3vXA+Lj1oesoBXm6iHEH1ydOe3ByylMClE6FKl9qDPMlB81iUxeAmqpXWblhFQ== X-Received: by 2002:a2e:88cb:: with SMTP id a11mr1059943ljk.304.1602068569184; Wed, 07 Oct 2020 04:02:49 -0700 (PDT) Received: from pc636 (h5ef52e31.seluork.dyn.perspektivbredband.net. [94.245.46.49]) by smtp.gmail.com with ESMTPSA id s17sm282520lfp.292.2020.10.07.04.02.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Oct 2020 04:02:48 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 7 Oct 2020 13:02:46 +0200 To: Michal Hocko Cc: Uladzislau Rezki , Mel Gorman , Vlastimil Babka , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , Peter Zijlstra , 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: <20201007110246.GA25550@pc636> References: <20200929220742.GB8768@pc636> <795d6aea-1846-6e08-ac1b-dbff82dd7133@suse.cz> <20201001192626.GA29606@pc636> <20201002071123.GB20872@dhcp22.suse.cz> <20201002085014.GC3227@techsingularity.net> <20201002090507.GB4555@dhcp22.suse.cz> <20201005150801.GC17959@pc636> <20201005154100.GF4555@dhcp22.suse.cz> <20201006222529.GA23612@pc636> <20201007100234.GI29020@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201007100234.GI29020@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 07, 2020 at 12:02:34PM +0200, Michal Hocko wrote: > On Wed 07-10-20 00:25:29, Uladzislau Rezki wrote: > > On Mon, Oct 05, 2020 at 05:41:00PM +0200, Michal Hocko wrote: > > > On Mon 05-10-20 17:08:01, Uladzislau Rezki wrote: > > > > On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote: > > > > > On Fri 02-10-20 09:50:14, Mel Gorman wrote: > > > > > > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote: > > > > > > > On Thu 01-10-20 21:26:26, 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 > > > > > > > > > > > > > > Even if a new gfp flag gains a sufficient traction and support I am > > > > > > > _strongly_ opposed against consuming another flag for that. Bit space is > > > > > > > limited. > > > > > > > > > > > > That is definitely true. I'm not happy with the GFP flag at all, the > > > > > > comment is at best a damage limiting move. It still would be better for > > > > > > a memory pool to be reserved and sized for critical allocations. > > > > > > > > > > Completely agreed. The only existing usecase is so special cased that a > > > > > dedicated pool is not only easier to maintain but it should be also much > > > > > better tuned for the specific workload. Something not really feasible > > > > > with the allocator. > > > > > > > > > > > > Besides that we certainly do not want to allow craziness like > > > > > > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we? > > > > > > > > > > > > That would deserve to be taken to a dumpster and set on fire. The flag > > > > > > combination could be checked in the allocator but the allocator path fast > > > > > > paths are bad enough already. > > > > > > > > > > If a new allocation/gfp mode is absolutely necessary then I believe that > > > > > the most reasoanble way forward would be > > > > > #define GFP_NO_LOCK ((__force gfp_t)0) > > > > > > > > > Agree. Even though i see that some code should be adjusted for it. There are > > > > a few users of the __get_free_page(0); So, need to double check it: > > > > > > Yes, I believe I have pointed that out in the previous discussion. > > > > > OK. I spent more time on it. A passed gfp_mask can be adjusted on the entry, > > that adjustment depends on the gfp_allowed_mask. It can be changed in run-time. > > > > For example during early boot it excludes: __GFP_RECLAIM|__GFP_IO|__GFP_FS flags, > > what is GFP_KERNEL. So, GFP_KERNEL is adjusted on entry and becomes 0 during early > > boot phase. > > Honestly I am not sure how much is GFP_BOOT_MASK still needed. The > remaining user of gfp_allowed_mask mask should be only hibernation and I > believe this should be removed in long term. Not as trivial because > scope API cannot be used for that as it needs a global flag but this is > a gross hack that should be implemented differently. It is waiting on my > todo list but never got around to that. > > > How to distinguish it: > > > > > > + /* > > + * gfp_mask can become zero because gfp_allowed_mask changes in run-time. > > + */ > > + if (!gfp_mask) > > + alloc_flags |= ALLOC_NO_LOCKS; > > + > > gfp_mask &= gfp_allowed_mask; > > alloc_mask = gfp_mask; > > if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags)) > > > > > > > > > > > > Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page(). > > > > If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages(). > > > > __kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks > > > > what can be easily converted to raw variant. > > > > > > Yes, there are likely more surprises like that. I am not sure about > > > kasan, page owner (which depens on the stack unwinder) and others which > > > hook into this path. > > > > > I have checked kasan_alloc_pages(), kernel_poison_pages() both are OK, > > at least i did not find any locking there. As for set_page_owner(), it > > requires more attention, since it uses arch specific unwind logic. Though, > > i spent some time on it and so far have not noticed anything. > > stack depod depends on a lock IIRC. Anyway, this is just showing how > this is going to grow in complexity and make future additions harder. > A niche usecase is inducing an additional complexity for future > maintenance. > I agree regarding maintenance costs. That is what is hard to avoid. -- Vlad Rezki