SMR only helps you on a per-queue basis. I think maybe ideally there would be a single pctrie here and some other mechanism for efficiently tracking dirty bufs, but I don't know how realistic that is.

Probably the right fix for getblk GB_NOCREAT is to re-check with the lock if unlocked lookup returned NULL. Something like:

	bp = gbincore_unlocked(bo, blkno);
	if (bp == NULL) {
		if (flags & GB_NOCREAT)
			goto loop;
		else
			goto newbuf_unlocked;
	}

(In the !GB_NOCREAT case, we might see the same race from gbincore_unlocked, but it is harmless: we'd create a useless buf at the same lbn, re-lock to attempt to insert it, and discover we'd lost the race. That race can happen even without SMR-gbincore.)

Ah, cool. I hope Suraj is doing well. Spinning locklessly again (GB_NOCREAT case) might be worth doing, but you'll still need the same locked fallback if you continue to get NULL (for correctness). If you should spin, and for how long, probably depends on how many GB_NOCREAT getblk requests will already exist in the bufq and be transitioning queues, vs true-positive misses in your workload. (And relative costs of a few lockless rechecks vs taking the bufobj lock.)