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([2600:1003:b867:fb88:f93f:7d9f:f5a9:d91e]) by smtp.gmail.com with ESMTPSA id e2sm619784qkg.77.2020.04.01.04.14.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 Apr 2020 04:14:18 -0700 (PDT) Date: Wed, 01 Apr 2020 07:14:16 -0400 User-Agent: K-9 Mail for Android In-Reply-To: <20200401072359.GC22681@dhcp22.suse.cz> References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331145806.GB236678@google.com> <20200331153450.GM30449@dhcp22.suse.cz> <20200331160117.GA170994@google.com> <20200401072359.GC22681@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern To: Michal Hocko CC: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org, neilb@suse.com, vbabka@suse.cz, mgorman@suse.de, Andrew Morton , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , Steven Rostedt From: joel@joelfernandes.org Message-ID: <30295C90-34DB-469C-9DCD-55DB91938BA9@joelfernandes.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On April 1, 2020 3:23:59 AM EDT, Michal Hocko wrote: >On Tue 31-03-20 12:01:17, Joel Fernandes wrote: >> On Tue, Mar 31, 2020 at 05:34:50PM +0200, Michal Hocko wrote: >> > On Tue 31-03-20 10:58:06, Joel Fernandes wrote: >> > [=2E=2E=2E] >> > > > diff --git a/kernel/rcu/tree=2Ec b/kernel/rcu/tree=2Ec >> > > > index 4be763355c9fb=2E=2E965deefffdd58 100644 >> > > > --- a/kernel/rcu/tree=2Ec >> > > > +++ b/kernel/rcu/tree=2Ec >> > > > @@ -3149,7 +3149,7 @@ static inline struct rcu_head >*attach_rcu_head_to_object(void *obj) >> > > > =20 >> > > > if (!ptr) >> > > > ptr =3D kmalloc(sizeof(unsigned long *) + >> > > > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN); >> > > > + sizeof(struct rcu_head), GFP_MEMALLOC); >> > >=20 >> > > Just to add, the main requirements here are: >> > > 1=2E Allocation should be bounded in time=2E >> > > 2=2E Allocation should try hard (possibly tapping into reserves) >> > > 3=2E Sleeping is Ok but should not affect the time bound=2E >> >=20 >> >=20 >> > __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to >> > memory reserves regarless of the sleeping status=2E >> >=20 >> > Using __GFP_MEMALLOC is quite dangerous because it can deplete >_all_ the >> > memory=2E What does prevent the above code path to do that? > >Neil has provided a nice explanation down the email thread=2E But let me >clarify few things here=2E > >> Can you suggest what prevents other users of GFP_MEMALLOC from doing >that >> also?=20 > >There is no explicit mechanism which is indeed unfortunate=2E The only >user real user of the flag is Swap over NFS AFAIK=2E I have never dared >to >look into details on how the complete reserves depletion is prevented=2E >Mel would be much better fit here=2E > >> That's the whole point of having a reserve, in normal usage no one >will >> use it, but some times you need to use it=2E Keep in mind this is not a >common >> case in this code here, this is triggered only if earlier allocation >attempts >> failed=2E Only *then* we try with GFP_MEMALLOC with promises to free >additional >> memory soon=2E > >You are right that this is the usecase for the flag=2E But this should be >done with an extreme care because the core MM relies on those reserves >so any other users should better make sure they do not consume a lot >from reserves as well=2E=20 > Understood and agreed=2E >> > If a partial access to reserves is sufficient then why the existing >> > modifiers (mentioned above are not sufficient? >>=20 >> The point with using GFP_MEMALLOC is it is useful for situations >where you >> are about to free memory and needed some memory temporarily, to free >that=2E It >> depletes it a bit temporarily to free even more=2E Is that not the >point of >> PF_MEMALLOC? >> * %__GFP_MEMALLOC allows access to all memory=2E This should only be >used when >> * the caller guarantees the allocation will allow more memory to be >freed >> * very shortly e=2Eg=2E process exiting or swapping=2E Users either sh= ould >> * be the MM or co-ordinating closely with the VM (e=2Eg=2E swap over >NFS)=2E >>=20 >> I was just recommending usage of this flag here because it fits the >> requirement of allocating some memory to free some memory=2E I am also >Ok with >> GFP_ATOMIC with the GFP_NOWARN removed, if you are Ok with that=2E > >Maybe we need to refine this documentation to be more explicit about an >extreme care to be taken when using the flag=2E > >diff --git a/include/linux/gfp=2Eh b/include/linux/gfp=2Eh >index e5b817cb86e7=2E=2Ee436a7e28392 100644 >--- a/include/linux/gfp=2Eh >+++ b/include/linux/gfp=2Eh >@@ -110,6 +110,9 @@ struct vm_area_struct; >* the caller guarantees the allocation will allow more memory to be >freed > * very shortly e=2Eg=2E process exiting or swapping=2E Users either sho= uld > * be the MM or co-ordinating closely with the VM (e=2Eg=2E swap over NFS= )=2E >+ * Users of this flag have to be extremely careful to not deplete the >reserve >+ * completely and implement a throttling mechanism which controls the >consumption >+ * based on the amount of freed memory=2E > * >* %__GFP_NOMEMALLOC is used to explicitly forbid access to emergency >reserves=2E > * This takes precedence over the %__GFP_MEMALLOC flag if both are set=2E I am in support of this documentation patch=2E I would say "consumption of= the reserve"=2E Thanks, - Joel --=20 Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E