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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id w29sm9705366lfq.35.2020.05.04.05.56.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 May 2020 05:56:26 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 4 May 2020 14:56:23 +0200 To: Joel Fernandes Cc: "Paul E. McKenney" , "Uladzislau Rezki (Sony)" , LKML , linux-mm@kvack.org, Andrew Morton , "Theodore Y . Ts'o" , Matthew Wilcox , RCU , Oleksiy Avramchenko Subject: Re: [PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment Message-ID: <20200504125623.GE17577@pc636> References: <20200428205903.61704-1-urezki@gmail.com> <20200428205903.61704-21-urezki@gmail.com> <20200501230052.GG7560@paulmck-ThinkPad-P72> <20200504002437.GA212435@google.com> <20200504002947.GG2869@paulmck-ThinkPad-P72> <20200504003106.GC212435@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200504003106.GC212435@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, May 03, 2020 at 08:31:06PM -0400, Joel Fernandes wrote: > On Sun, May 03, 2020 at 05:29:47PM -0700, Paul E. McKenney wrote: > > On Sun, May 03, 2020 at 08:24:37PM -0400, Joel Fernandes wrote: > > > On Fri, May 01, 2020 at 04:00:52PM -0700, Paul E. McKenney wrote: > > > > On Tue, Apr 28, 2020 at 10:58:59PM +0200, Uladzislau Rezki (Sony) wrote: > > > > > From: "Joel Fernandes (Google)" > > > > > > > > > > Handle cases where the the object being kvfree_rcu()'d is not aligned by > > > > > 2-byte boundaries. > > > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > > > Signed-off-by: Joel Fernandes (Google) > > > > > --- > > > > > kernel/rcu/tree.c | 9 ++++++--- > > > > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > index 501cac02146d..649bad7ad0f0 100644 > > > > > --- a/kernel/rcu/tree.c > > > > > +++ b/kernel/rcu/tree.c > > > > > @@ -2877,6 +2877,9 @@ struct kvfree_rcu_bulk_data { > > > > > #define KVFREE_BULK_MAX_ENTR \ > > > > > ((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *)) > > > > > > > > > > +/* Encoding the offset of a fake rcu_head to indicate the head is a wrapper. */ > > > > > +#define RCU_HEADLESS_KFREE BIT(31) > > > > > > > > Did I miss the check for freeing something larger than 2GB? Or is this > > > > impossible, even on systems with many terabytes of physical memory? > > > > Even if it is currently impossible, what prevents it from suddenly > > > > becoming all too possible at some random point in the future? If you > > > > think that this will never happen, please keep in mind that the first > > > > time I heard "640K ought to be enough for anybody", it sounded eminently > > > > reasonable to me. > > > > > > > > Besides... > > > > > > > > Isn't the offset in question the offset of an rcu_head struct within > > > > the enclosing structure? If so, why not keep the current requirement > > > > that this be at least 16-bit aligned, especially given that some work > > > > is required to make that alignment less than pointer sized? Then you > > > > can continue using bit 0. > > > > > > > > This alignment requirement is included in the RCU requirements > > > > documentation and is enforced within the __call_rcu() function. > > > > > > > > So let's leave this at bit 0. > > > > > > This patch is needed only if we are growing the fake rcu_head. Since you > > > mentioned in a previous patch in this series that you don't want to do that, > > > and just rely on availability of the array of pointers or synchronize_rcu(), > > > we can drop this patch. If we are not dropping that earlier patch, let us > > > discuss more. > > > > Dropping it sounds very good to me! > > Cool ;-) Thanks, > OK. Then we drop this patch and all dynamic rcu_head attaching logic what will make the code size smaller. Thanks! -- Vlad Rezki