From: Matthew Wilcox
From: Rasmus Villemoes [mailto:linux@rasmusvillemoes.dk]
This sounds good. I think there may still be a lot of users that never allocate more than a handful of IDAs, making a 128 byte allocation still somewhat excessive. One thing I considered was (exactly as it's done for file descriptor tables) to embed a single word in the struct ida and use that initially; I haven't looked closely at newIDA, so I don't know how easy that would be or if its worth the complexity.
Heh, I was thinking about that too. The radix tree supports "exceptional entries" which have the bottom bit set. On a 64-bit machine, we could use 62 of the bits in the radix tree root to store the ID bitmap. I'm a little wary of the potential complexity, but we should try it out.
Test patch here: http://git.infradead.org/users/willy/linux-dax.git/shortlog/refs/heads/idr-2... It passes the test suite ... which I actually had to adjust because it now succeeds in cases where it hadn't (allocating ID 0 without preallocating), and it will now fail in cases where it hadn't previously (assuming a single preallocation would be enough). There shouldn't be any examples of that in the kernel proper; it was simply me being lazy when I wrote the test suite.