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

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Date:   Wed, 7 Feb 2018 08:18:46 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, 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, brouer@...hat.com, rao.shoaib@...cle.com
Subject: Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree_rcu()

On Wed, Feb 07, 2018 at 08:57:00AM -0500, Steven Rostedt 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

You missed kvmalloc() ...

> 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.

I'd still like it to be called free_rcu() ... so let's turn it around.

What memory can you allocate and then *not* free by calling kvfree()?
kvfree() can free memory allocated by kmalloc(), vmalloc(), any slab
allocation (is that guaranteed, or just something that happens to work?)
I think it can't free per-cpu allocations, bootmem, DMA allocations, or
alloc_page/get_free_page.

Do we need to be able to free any of those objects in order to rename
kfree_rcu() to just free_rcu()?