Re: [PATCH 1/12] radix_tree: exceptional entries and indices
From: Randy Dunlap
Date: Fri Jun 17 2011 - 20:12:34 EST
On Fri, 17 Jun 2011 17:07:42 -0700 Randy Dunlap wrote:
> 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@xxxxxxxxxx> 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
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