Re: [patch] mm: fix anon_vma races

Hugh Dickins
2008-10-20T03:26:39Z

On Sun, 19 Oct 2008, Hugh Dickins wrote:
> On Sun, 19 Oct 2008, Nick Piggin wrote:
> >
> > There is already a page_mapped check in there. I'm just going to
> > propose we move that down. No extra branchesin the fastpath. OK?
>
> That should be OK, yes. Looking back at the history, I believe
> I sited the page_mapped test where it is, partly for simpler flow,
> and partly to avoid overhead of taking spinlock unnecessarily.

Arrgh! What terrible advice I gave you there, completely wrong:
that's what happens when I rush a reply instead of thinking.

I'm three-quarters through replying to Linus on this, and going
into that detail, remember now why its placement is critical.

Repeat the page_mapped check before returning if you wish,
but do not remove the one that's there: see other mail for
explanation.

Hugh