linux-kernel - Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree

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Message-ID: <20180207174513.5cc9b503@redhat.com>
Date:   Wed, 7 Feb 2018 17:45:13 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Matthew Wilcox <willy@...radead.org>, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        mingo@...hat.com, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rao.shoaib@...cle.com, brouer@...hat.com
Subject: Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree_rcu()

On Wed, 7 Feb 2018 08:57:00 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 7 Feb 2018 00:31:04 -0800
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > I see problems.  We would then have two different names for exactly the
> > same thing.
> > 
> > Seems like it would be a lot easier to simply document the existing
> > kfree_rcu() behavior, especially given that it apparently already works.
> > The really doesn't seem to me to be worth a name change.  
> 
> Honestly, I don't believe this is an RCU sub-system decision. This is a
> memory management decision.
> 
> If we have kmalloc(), vmalloc(), kfree(), vfree() and kvfree(), and we
> want kmalloc() to be freed with kfree(), and vmalloc() to be freed with
> vfree(), and for strange reasons, we don't know how the data was
> allocated we have kvfree(). That's an mm decision not an rcu one. We
> should have kfree_rcu(), vfree_rcu() and kvfree_rcu(), and honestly,
> they should not depend on kvfree() doing the same thing for everything.
> Each should call the corresponding member that they represent. Which
> would change this patch set.
> 
> Why? Too much coupling between RCU and MM. What if in the future
> something changes and kvfree() goes away or changes drastically. We
> would then have to go through all the users of RCU to change them too.
> 
> To me kvfree() is a special case and should not be used by RCU as a
> generic function. That would make RCU and MM much more coupled than
> necessary.

For the record, I fully agree with Steve here. 

And being a performance "fanatic" I don't like to have the extra branch
(and compares) in the free code path... but it's a MM-decision (and
sometimes you should not listen to "fanatics" ;-))

void kvfree(const void *addr)
{
	if (is_vmalloc_addr(addr))
		vfree(addr);
	else
		kfree(addr);
}
EXPORT_SYMBOL(kvfree);

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer


static inline bool is_vmalloc_addr(const void *x)
{
#ifdef CONFIG_MMU
	unsigned long addr = (unsigned long)x;

	return addr >= VMALLOC_START && addr < VMALLOC_END;
#else
	return false;
#endif
}