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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id j125sm8678528lfj.38.2020.03.31.07.04.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2020 07:04:40 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 31 Mar 2020 16:04:33 +0200 To: "Joel Fernandes (Google)" 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 Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Message-ID: <20200331140433.GA26498@pc636> References: <20200331131628.153118-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200331131628.153118-1-joel@joelfernandes.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 31, 2020 at 09:16:28AM -0400, Joel Fernandes (Google) wrote: > In kfree_rcu() headless implementation (where the caller need not pass > an rcu_head, but rather directly pass a pointer to an object), we have > a fall-back where we allocate a rcu_head wrapper for the caller (not the > common case). This brings the pattern of needing to allocate some memory > to free some memory. Currently we use GFP_ATOMIC flag to try harder for > this allocation, however the GFP_MEMALLOC flag is more tailored to this > pattern. We need to try harder not only during atomic context, but also > during non-atomic context anyway. So use the GFP_MEMALLOC flag instead. > > Also remove the __GFP_NOWARN flag simply because although we do have a > synchronize_rcu() fallback for absolutely worst case, we still would > like to not enter that path and atleast trigger a warning to the user. > > Cc: linux-mm@kvack.org > Cc: rcu@vger.kernel.org > Cc: willy@infradead.org > Cc: peterz@infradead.org > Cc: neilb@suse.com > Cc: vbabka@suse.cz > Cc: mgorman@suse.de > Cc: Andrew Morton > Signed-off-by: Joel Fernandes (Google) > --- > > This patch is based on the (not yet upstream) code in: > git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree) > > It is a follow-up to the posted series: > https://lore.kernel.org/lkml/20200330023248.164994-1-joel@joelfernandes.org/ > > > kernel/rcu/tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 4be763355c9fb..965deefffdd58 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj) > > if (!ptr) > ptr = kmalloc(sizeof(unsigned long *) + > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN); > + sizeof(struct rcu_head), GFP_MEMALLOC); > Hello, Joel I have some questions regarding improving it, see below them: Do you mean __GFP_MEMALLOC? Can that flag be used in atomic context? Actually we do allocate there under spin lock. Should be combined with GFP_ATOMIC | __GFP_MEMALLOC? As for removing __GFP_NOWARN. Actually it is expectable that an allocation can fail, if so we follow last emergency case. You can see the trace but what would you do with that information? Thanks! -- Vlad Rezki