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[37.188.180.223]) by smtp.gmail.com with ESMTPSA id t10sm2651323wrx.38.2020.04.01.06.15.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Apr 2020 06:15:30 -0700 (PDT) Date: Wed, 1 Apr 2020 15:15:28 +0200 From: Michal Hocko To: Uladzislau Rezki Cc: Joel Fernandes , 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 Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Message-ID: <20200401131528.GK22681@dhcp22.suse.cz> References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331145806.GB236678@google.com> <20200331153450.GM30449@dhcp22.suse.cz> <20200331161215.GA27676@pc636> <20200401070958.GB22681@dhcp22.suse.cz> <20200401123230.GB32593@pc636> <20200401125503.GJ22681@dhcp22.suse.cz> <20200401130816.GA1320@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200401130816.GA1320@pc636> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 01-04-20 15:08:16, Uladzislau Rezki wrote: > On Wed, Apr 01, 2020 at 02:55:03PM +0200, Michal Hocko wrote: > > On Wed 01-04-20 14:32:30, Uladzislau Rezki wrote: > > > On Wed, Apr 01, 2020 at 09:09:58AM +0200, Michal Hocko wrote: > > > > On Tue 31-03-20 18:12:15, Uladzislau Rezki wrote: > > > > > > > > > > > > __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to > > > > > > memory reserves regarless of the sleeping status. > > > > > > > > > > > Michal, just one question here regarding proposed flags. Can we also > > > > > tight it with __GFP_RETRY_MAYFAIL flag? Means it also can repeat a few > > > > > times in order to increase the chance of being success. > > > > > > > > yes, __GFP_RETRY_MAYFAIL is perfectly valid with __GFP_ATOMIC. Please > > > > note that __GFP_ATOMIC, despite its name, doesn't imply an atomic > > > > allocation which cannot sleep. Quite confusing, I know. A much better > > > > name would be __GFP_RESERVES or something like that. > > > > > > > OK. Then we can use GFP_ATOMIC | __GFP_RETRY_MAYFAIL to try in more harder > > > way. > > > > Please note the difference between __GFP_ATOMIC and GFP_ATOMIC. The > > later is a highlevel flag to use for atomic contexts. The former is an > > explicit way to give an access to memory reserves. I am not familiar > > with your code but if you have an existing gfp context coming from the > > caller then just do (gfp | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL). > > If you do not have any gfp then decide based on whether the current > > context is allowed to sleep > > gfp = GFP_KERNEL | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL; > > if (!sleepable) > > gfp &= ~__GFP_DIRECT_RECLAIM; > > We call it from atomic context, so we can not sleep, also we do not have > any existing context coming from the caller. I see that GFP_ATOMIC is high-level > flag and is differ from __GFP_ATOMIC. It is defined as: > > #define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM) > > so basically we would like to have __GFP_KSWAPD_RECLAIM that is included in it, > because it will also help in case of high memory pressure and wake-up kswapd to > reclaim memory. > > We also can extract: > > __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL | __GFP_KSWAPD_RECLAIM > > but that is longer then > > GFP_ATMOC | __GFP_RETRY_MAYFAIL OK, if you are always in the atomic context then GFP_ATOMIC is sufficient. __GFP_RETRY_MAYFAIL will make no difference for allocations which do not reclaim (and thus not retry). Sorry this was not clear to me from the previous description. -- Michal Hocko SUSE Labs