From: Matthew Wilcox
From: Matthew Wilcox
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-2016-12-16 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.
Found a bug, committed a fix (and another test case). It now no longer returns -EAGAIN in situations where it wouldn't have, so I've reverted that bit of the test suite change. Still succeeds when it wouldn't have before, but I feel no pressure to change that ;-)