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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id q24sm3289429lfo.149.2020.09.29.09.25.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Sep 2020 09:25:16 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 29 Sep 2020 18:25:14 +0200 To: Michal Hocko Cc: Uladzislau Rezki , Mel Gorman , "Paul E. McKenney" , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Peter Zijlstra , Vlastimil Babka , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Mel Gorman Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20200929162514.GA8768@pc636> References: <20200921194819.GA24236@pc636> <20200922075002.GU12990@dhcp22.suse.cz> <20200922131257.GA29241@pc636> <20200923103706.GJ3179@techsingularity.net> <20200923154105.GO29330@paulmck-ThinkPad-P72> <20200923232251.GK3179@techsingularity.net> <20200924081614.GA14819@pc636> <20200925080503.GC3389@dhcp22.suse.cz> <20200925153129.GB25350@pc636> <20200925154741.GI3389@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200925154741.GI3389@dhcp22.suse.cz> 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: > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability > > to provide a memory service for contexts which are not allowed to > > sleep, RCU is part of them. Both flags used to provide such ability > > before but not anymore. > > > > Do you agree with it? > > Yes this sucks. But this is something that we likely really want to live > with. We have to explicitly _document_ that really atomic contexts in RT > cannot use the allocator. From the past discussions we've had this is > likely the most reasonable way forward because we do not really want to > encourage anybody to do something like that and there should be ways > around that. The same is btw. true also for !RT. The allocator is not > NMI safe and while we should be able to make it compatible I am not > convinced we really want to. > > Would something like this be helpful wrt documentation? > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > index 67a0774e080b..9fcd47606493 100644 > --- a/include/linux/gfp.h > +++ b/include/linux/gfp.h > @@ -238,7 +238,9 @@ struct vm_area_struct; > * %__GFP_FOO flags as necessary. > * > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > - * watermark is applied to allow access to "atomic reserves" > + * watermark is applied to allow access to "atomic reserves". > + * The current implementation doesn't support NMI and other non-preemptive context > + * (e.g. raw_spin_lock). > * > * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires > * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim. > To me it is clear. But also above conflicting statement: %GFP_ATOMIC users can not sleep and need the allocation to succeed. A %lower should be rephrased, IMHO. -- Vlad Rezki