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[23.128.96.18]) by mx.google.com with ESMTP id x93si531352ede.583.2020.10.02.02.09.15; Fri, 02 Oct 2020 02:09:37 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@suse.com header.s=susede1 header.b=nBaClv4g; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=suse.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387654AbgJBJFN (ORCPT + 99 others); Fri, 2 Oct 2020 05:05:13 -0400 Received: from mx2.suse.de ([195.135.220.15]:51324 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726029AbgJBJFK (ORCPT ); Fri, 2 Oct 2020 05:05:10 -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=1601629508; h=from:from:reply-to:subject:subject: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=NLWsHd/XYO6vX/kTuPru1/vkApAYvvP2ucBNJ3kKVJc=; b=nBaClv4gbAC5DRD/cBtVyI8bqQREp2ba72exdgoEbhBJpsfwpn/ZoB3oPxLsMABwnqlpRu HzEBZxVHxXQrmsKtRjP0XMUiPs1bX615oSEwHpwz9Nb/lecPKLCOrIUClTK6lUaxeOxGqW i/MSW4kEL6K8x/sZh3QhX/3UrPsQ7qE= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 4F805AC82; Fri, 2 Oct 2020 09:05:08 +0000 (UTC) Date: Fri, 2 Oct 2020 11:05:07 +0200 From: Michal Hocko To: Mel Gorman Cc: Uladzislau Rezki , 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: <20201002090507.GB4555@dhcp22.suse.cz> 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> <20201002071123.GB20872@dhcp22.suse.cz> <20201002085014.GC3227@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201002085014.GC3227@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 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) and explicitly document it as a final flag to use without any further modifiers. Yeah there are some that could be used potentially - e.g. zone specifiers, __GFP_ZERO and likely few others. But support for those can be added when there is an actual and reasonable demand. I would also strongly argue against implementation alowing to fully consume pcp free pages. -- Michal Hocko SUSE Labs