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 :( -- 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>