Use enumeration to indicate swap almost full and full, instead of 2 variables.

[ota_j.email.ne.jp]

Summary

Use enumeration to indicate usage level of swap space and simplify code.

Both high and low watermark are too low at today's hardware, like 1 ~ 10 GB
while it was large enough numbers 20 years ago 16 ~ 64 MB.
I'd like to propose to change 1/16 as high watermark and 1/32 as low one
to adjust at swapon time (as a separate and next changeset.)

Test Plan

Use all swap spaces to trigger "swap_pager: almost out of swap" print out.

[dougm]

sys/vm/swap_pager.c

482–485

How about "== SWPG_LOW"?

489

Why is this line necessary?

768

Does this line really compile? It looks about 3 arguments short.

[ota_j.email.ne.jp]

Use "== SWPG_LOW" comparison for almost full warning.
Drop swp_gauge > SWPG_LOW comparison for high watermark check.

[dougm]

sys/vm/swap_pager.c

488

How about if swap_pager_avail >= nswap_lowat and swap_pager_avail <= nswap_hiwat? You are changing that behavior.

[ota_j.email.ne.jp]

sys/vm/swap_pager.c

488

That's a case when we reset SWPG_FULL status.

Page in/out doesn't happen over (512-128) size block and system will hit the other cases.
So, I think in order to hit this range with SWPG_FULL, one needs to swap on/off a couple of MB swap devices.
Given today's physical memory is in the unit of GB, a couple of MB swap won't save the system.

I think we are okay as is for now.

Or I can add else case to the top if-elseif like following:
else {

if(swp_gauge == SWPG_FULL)
    swp_gauge = SWPG_LOW;

}

What do you think?

[freqlabs]

sys/vm/swap_pager.c

326

Please add comments describing what each of these means.