Re: [PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment

[Joel Fernandes]

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@xxxxxxxxxxxxxxxxx>
> > 
> > Handle cases where the the object being kvfree_rcu()'d is not aligned by
> > 2-byte boundaries.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  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