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=-5.2 required=3.0 tests=BAYES_00, 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 99B11C47423 for ; Fri, 2 Oct 2020 08:06:31 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 411B8206B6 for ; Fri, 2 Oct 2020 08:06:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 411B8206B6 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=techsingularity.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D5D346B0068; Fri, 2 Oct 2020 04:06:30 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D0E1C8E0001; Fri, 2 Oct 2020 04:06:30 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C4AE06B006E; Fri, 2 Oct 2020 04:06:30 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0237.hostedemail.com [216.40.44.237]) by kanga.kvack.org (Postfix) with ESMTP id 98B406B0068 for ; Fri, 2 Oct 2020 04:06:30 -0400 (EDT) Received: from smtpin10.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 3B9598249980 for ; Fri, 2 Oct 2020 08:06:30 +0000 (UTC) X-FDA: 77326253340.10.dock61_24076b3271a2 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin10.hostedemail.com (Postfix) with ESMTP id 1BF2916A0DE for ; Fri, 2 Oct 2020 08:06:30 +0000 (UTC) X-HE-Tag: dock61_24076b3271a2 X-Filterd-Recvd-Size: 5405 Received: from outbound-smtp09.blacknight.com (outbound-smtp09.blacknight.com [46.22.139.14]) by imf46.hostedemail.com (Postfix) with ESMTP for ; Fri, 2 Oct 2020 08:06:29 +0000 (UTC) Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp09.blacknight.com (Postfix) with ESMTPS id B9E0A1C39D1 for ; Fri, 2 Oct 2020 09:06:27 +0100 (IST) Received: (qmail 20516 invoked from network); 2 Oct 2020 08:06:27 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 2 Oct 2020 08:06:27 -0000 Date: Fri, 2 Oct 2020 09:06:24 +0100 From: Mel Gorman To: Uladzislau Rezki Cc: 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: <20201002080624.GB3227@techsingularity.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20201001192626.GA29606@pc636> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. For anyone reviewing, any use of __GFP_NO_LOCKS should meet a high standard where there is no alternative except to use the flags. i.e. a higher standard "but I'm an important driver". -- Mel Gorman SUSE Labs