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 http://userweb.kernel.org/~akpm/stuff/rtth.tar.gz --- ~Randy *** Remember to use Documentation/SubmitChecklist when testing your code *** -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>