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

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Message-Id: <20110618144832.cfc665b0.akpm@linux-foundation.org>
Date:	Sat, 18 Jun 2011 14:48:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/12] radix_tree: exceptional entries and indices

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

> On Fri, 17 Jun 2011, Andrew Morton wrote:
> > On Tue, 14 Jun 2011 03:42:27 -0700 (PDT)
> > Hugh Dickins <hughd@...gle.com> 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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