<tries to remember what this is all about> l 2011 15:56:14 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote: > On Sat, 18 Jun 2011, Andrew Morton wrote: > > 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. > > I don't think that remark is fair to lockless pagecache at all. If we > want the scalability advantage of lockless lookup, yes, we don't have > strict coherency with tagging at that time. But those places that need > to worry about that coherency, can lock to do so. Nobody thought about these issues, afaik. Things have broken and the code has become significantly more complex/fragile. Does the locking in mapping_tagged() make any sense? > > 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? > > I wouldn't call that an improvement... I wouldn't call the proposed changes to radix-tree.c an improvement, either. It's an expedient, once-off, single-caller hack. If the cost of adding locking is negligible then that is a superior fix. > > What effect does that have? > > ... but admit I have not measured: I rather assume that if we now change > tmpfs from lockless to locked lookup, someone else will soon come up with > the regression numbers. > > > It'd better be > > "really bad", because this patchset does nothing at all to improve core > > MM maintainability :( > > I was aiming to improve shmem.c maintainability; and you have good grounds > to accuse me of hurting shmem.c maintainability when I highmem-ized the > swap vector nine years ago. > > I was not aiming to improve core MM maintainability, nor to harm it. > I am extending the use to which the radix-tree can be put, but is that > so bad? I find it hard to believe that this wart added to the side of the radix-tree code will find any other users. And the wart spreads contagion into core filemap pagecache lookup. It's pretty nasty stuff. Please, what is a better way of doing all this? -- 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>