Fix an edge case in the node count computation in blist_create().

[markj]

Summary

There are some cases where we need to allocate an extra terminator node
even when last_block == blocks - 1. This occurs when each leaf is
completely filled out (i.e., blocks is a sum of powers of 2 greater than
or equal to BLIST_BMAP_RADIX) but "blocks" is smaller than "radix".

While here, add an explicit check for the blocks == 0 case, which
is otherwise invalid and not handled properly.

Test Plan

Compiled subr_blist.c for userland and verified that we allocate the
correct number of nodes and place terminators at the correct offsets
for various inputs to blist_create().

[markj]

sys/kern/subr_blist.c

252 ↗

(On Diff #47648)

Oops, the last condition is redundant. It's sufficient to check radix - blocks > BLIST_BMAP_RADIX.

[markj]

sys/kern/subr_blist.c

251 ↗

(On Diff #47648)

The comparison should also be >=, not >.

[markj]

• Address comments.

[dougm]

sys/kern/subr_blist.c

251 ↗

(On Diff #47650)

I read this as "If there are missing leaf (bmap) nodes that keep this from being a complete balanced tree, then we need a terminator somewhere. I agree. But then does the "last_block >= blocks" test still serve any purpose? Perhaps even the iterative calculation of last_block should be dropped?

[markj]

• Simplify the test for the presence of a terminator node.
• Update comments to reflect the fact that last_block is not used in the above-mentioned test.

[markj]

sys/kern/subr_blist.c

251 ↗

(On Diff #47650)

I believe you're right that the last_block >= blocks tests is not needed anymore, but we still need the iterative computation of last_block in order for the node count computation below to be correct. I don't really see a way to compute both "nodes" and "radix" with a single loop - do you?