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Date: Sun, 3 May 2020 20:24:37 -0400 From: Joel Fernandes <joel@...lfernandes.org> To: "Paul E. McKenney" <paulmck@...nel.org> Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, "Theodore Y . Ts'o" <tytso@....edu>, Matthew Wilcox <willy@...radead.org>, RCU <rcu@...r.kernel.org>, Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com> Subject: Re: [PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment 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)" <joel@...lfernandes.org> > > > > Handle cases where the the object being kvfree_rcu()'d is not aligned by > > 2-byte boundaries. > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org> > > --- > > 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. thanks, - Joel
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