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Date: Fri, 17 Jun 2011 17:07:42 -0700
From: Randy Dunlap <rdunlap@xenotime.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 1/12] radix_tree: exceptional entries and indices
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On Fri, 17 Jun 2011 16:38:54 -0700 Andrew Morton wrote:

> On Tue, 14 Jun 2011 03:42:27 -0700 (PDT)
> Hugh Dickins <hughd@google.com> wrote:
> 
> > The radix_tree is used by several subsystems for different purposes.
> > A major use is to store the struct page pointers of a file's pagecache
> > for memory management.  But what if mm wanted to store something other
> > than page pointers there too?
> > 
> > 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.
> > 
> > If a subsystem already uses radix_tree with that bit set, no problem:
> > it does not affect internal workings at all, but is defined for the
> > convenience of those storing well-aligned pointers in the radix_tree.
> > 
> > The radix_tree_gang_lookups have an implicit assumption that the caller
> > can deduce the offset of each entry returned e.g. by the page->index of
> > a struct page.  But that may not be feasible for some kinds of item to
> > be stored there.
> > 
> > radix_tree_gang_lookup_slot() allow for an optional indices argument,
> > output array in which to return those offsets.  The same could be added
> > to other radix_tree_gang_lookups, but for now keep it to the only one
> > for which we need it.
> 
> 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?


And regardless of the patch path that is taken, update test(s) if
applicable.  I thought that someone from Red Hat had a kernel loadable
module for testing radix-tree -- or maybe that was for rbtree (?) --
but I can't find that just now.

And one Andrew Morton has a userspace radix tree test harness at
http://userweb.kernel.org/~akpm/stuff/rtth.tar.gz

---
~Randy
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