vmem: Simplify bt_fill() callers a bit

[markj]

Summary

No functional change intended.

[alc]

sys/kern/subr_vmem.c

1523–1524

This function can be further simplified:

error = bt_fill(vm, flags);
if (error == 0)

[rlibby]

Why not just put the logic in bt_fill(), is there a future patch that plans to use _bt_fill()?

Or were you hoping to have bt_fill() inlined and _bt_fill() not inlined? If the latter, maybe consider adding the inline/noinline tags?

[alc]

sys/kern/subr_vmem.c

1411–1418

As an "oh, by the way," this code does not really implement "best fit". It selects the first available tag that is sufficiently large, not the best fit. The NetBSD version acknowledges this in a comment that didn't get brought over to FreeBSD:

		/*
		 * we assume that, for space efficiency, it's better to
		 * allocate from a smaller block.  thus we will start searching
		 * from the lower-order list than VM_INSTANTFIT.
		 * however, don't bother to find the smallest block in a free
		 * list because the list can be very long.  we can revisit it
		 * if/when it turns out to be a problem.
		 *
		 * note that the 'first' list can contain blocks smaller than
		 * the requested size.  thus we need to check bt_size.
		 */

When I first noticed this, I also looked at the old OpenSolaris sources. As per the earlier vmem paper, their implicit default is "instant fit", which behaves like our "first fit". Alternatively, you can explicitly request "first fit", which behaves like our "best fit", or "best fit", which we don't have. The earlier vmem paper didn't mention a "first fit" policy.

[markj]

In D26769#596914, @rlibby wrote:

Why not just put the logic in bt_fill(), is there a future patch that plans to use _bt_fill()?

Or were you hoping to have bt_fill() inlined and _bt_fill() not inlined? If the latter, maybe consider adding the inline/noinline tags?

Yeah, I wanted to inline bt_fill() but not _bt_fill() to preserve the old code layout. With some usage patterns vmem_xalloc() will hit the "slow" path most of the time, so I'm not sure it matters much.

[markj]

Address feedback.

[alc]

sys/kern/subr_vmem.c

1398–1401

error = bt_fill(vm, flags);
if (error != 0)
        break;

1520

This assignment is now redundant.

[markj]

• Address alc's feedback.