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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id b197sm304632lfd.251.2020.09.30.13.36.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Sep 2020 13:36:14 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 30 Sep 2020 22:36:12 +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: <20200930203612.GA22035@pc636> References: <20200924081614.GA14819@pc636> <20200925080503.GC3389@dhcp22.suse.cz> <20200925153129.GB25350@pc636> <20200925154741.GI3389@dhcp22.suse.cz> <20200929162514.GA8768@pc636> <20200930092732.GP2277@dhcp22.suse.cz> <20200930123535.GB18005@pc636> <20200930124413.GU2277@dhcp22.suse.cz> <20200930133954.GA19235@pc636> <20200930164600.GW2277@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200930164600.GW2277@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, Sep 30, 2020 at 06:46:00PM +0200, Michal Hocko wrote: > On Wed 30-09-20 15:39:54, Uladzislau Rezki wrote: > > On Wed, Sep 30, 2020 at 02:44:13PM +0200, Michal Hocko wrote: > > > On Wed 30-09-20 14:35:35, Uladzislau Rezki wrote: > > > > On Wed, Sep 30, 2020 at 11:27:32AM +0200, Michal Hocko wrote: > > > > > On Tue 29-09-20 18:25:14, Uladzislau Rezki wrote: > > > > > > > > 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. > > > > > > > > > > Any suggestions? Or more specifics about which part is conflicting? It > > > > > tries to say that there is a higher demand to succeed even though the > > > > > context cannot sleep to take active measures to achieve that. So the > > > > > only way to achieve that is to break the watermakrs to a certain degree > > > > > which is making them more "higher class" than other allocations. > > > > > > > > > Michal, i had only one concern about it. It says that %GFP_ATOMIC users > > > > can not sleep, i.e. callers know that they are in atomic, thus no any > > > > sleeping, but the chose they make will force them to sleep. > > > > > > I am not sure I follow you here. Do you mean they will be forced to > > > sleep with PREEMPT_RT? > > > > > Exactly :) > > We can make that more specific once RT patchset is merged. As of now > this is not the thing in the Linus tree. I believe there will be more to > clarify about atomic contexts in the RT tree as it means something else > than people are used to think. > Fair enough. Currently we have only the Kconfig CONFIG_PREEMPT_RT symbol, so RT patches are still not in place. -- Vlad Rezki