linux-kernel - Re: [PATCH 3/3] mm: Simplify anon

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Date:	Thu, 17 Feb 2011 10:30:07 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

On Thu, Feb 17, 2011 at 8:19 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> +void __put_anon_vma(struct anon_vma *anon_vma)
> +{
> +       if (anon_vma->root != anon_vma)
> +               put_anon_vma(anon_vma->root);
> +       anon_vma_free(anon_vma);
>  }

So this makes me nervous. It looks like recursion.

Now, I don't think we can ever get a chain of these things (because
the root should be the root of everything), but I still preferred the
older code that made that "one-level root" case explicit, and didn't
have recursion.

IOW, even though it should be entirely equivalent, I think I'd really
prefer something like

  void __put_anon_vma(struct anon_vma *anon_vma)
  {
    struct anon_vma *root = anon_vma->root;

    if (root != anon_vma && atomic_dec_and_test(&root->refcount))
      anon_vma_free(root);
    anon_vma_free(anon_vma);
  }

instead. Exactly because it makes it very clear that the "root" is a
root, and we're not doing some possibly arbitrarily deep list like the
dentry tree (which avoids recursion by open-coding its freeing as a
loop).

Hmm? (The above is obviously untested, maybe it has some stupid bug)

                   Linus
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