Re: [PATCH 1/12] radix_tree: exceptional entries and indices

From: Andrew Morton
Date: Sat Jun 18 2011 - 17:48:58 EST

On Fri, 17 Jun 2011 17:13:38 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote:

> On Fri, 17 Jun 2011, Andrew Morton wrote:
> > On Tue, 14 Jun 2011 03:42:27 -0700 (PDT)
> > Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > > The low bit of a radix_tree entry is already used to denote an indirect
> > > pointer, for internal use, and the unlikely radix_tree_deref_retry() case.
> > > Define the next bit as denoting an exceptional entry, and supply inline
> > > functions radix_tree_exception() to return non-0 in either unlikely case,
> > > and radix_tree_exceptional_entry() to return non-0 in the second case.
> >
> > Yes, the RADIX_TREE_INDIRECT_PTR hack is internal-use-only, and doesn't
> > operate on (and hence doesn't corrupt) client-provided items.
> >
> > This patch uses bit 1 and uses it against client items, so for
> > practical purpoese it can only be used when the client is storing
> > addresses. And it needs new APIs to access that flag.
> >
> > All a bit ugly. Why not just add another tag for this? Or reuse an
> > existing tag if the current tags aren't all used for these types of
> > pages?
> I couldn't see how to use tags without losing the "lockless" lookups:

So lockless pagecache broke the radix-tree tag-versus-item coherency as
well as the address_space nrpages-vs-radix-tree coherency. Isn't it
fun learning these things.

> because the tag is a separate bit from the entry itself, unless you're
> under tree_lock, there would be races when changing from page pointer
> to swap entry or back, when slot was updated but tag not or vice versa.

So... take tree_lock? What effect does that have? It'd better be
"really bad", because this patchset does nothing at all to improve core
MM maintainability :(

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