Use kmem_zalloc instead of kmem_alloc + bzero?

Spacing on =

Pretty sure this isn't needed given its a bzero under the covers and g_alloc_bio specifies M_ZERO.

Any reason why j++ isn't in the for?

This looks odd as error is loop invariant, bug?

Revision 295707 says we should always use g_reset_bio after g_alloc_bio because g_reset_bio might not always be bzero.

Nope.

Yes.

That sounds like a very odd change as double zeroing is quite an overhead, also doesn't correspond with what was committed as only existing bzero's where changed no addition of g_reset_bio after existing g_alloc_bio / g_new_bio.

If its required by those calls then they should do it, not rely on every single caller performing the two calls?

This call to g_reset_bios is bogus. You just allocated the bio, yo don't need to call it unless you *REUSE* the bio later.

Do I correctly parse this line as returning an overall failure when at least one command fails?
I think that that makes the code less robust: if reading any single of 4 vdev labels fails for whatever reason (e.g. failing hardware), then we would ignore the successfully read labels.

I'm afraid that you're correct. It's not completely straightforward to fix since there isn't a 1-1 relationship between commands and bios. I'll try to come up with a fix.