Don't update per-page activation counts in the swapout code.

[markj]

Summary

This is the work of the page daemon. Now that the page daemon does slow
and steady scanning of the active queue, I don't think it makes much
sense to replicate that algorithm here. Moreover, it was doing so
inconsistently:

• PGA_REFERENCED is not counted in act_count unless pmap_ts_referenced() returned 0, but the page daemon always counts PGA_REFERENCED.
• The swapout daemon always activates a referenced page, but the page daemon only does so when the containing object is mapped at least once.

The main purpose of swapout_deactivate_pages() is to shrink the number
of pages mapped into a given pmap. To do this without unmapping active
pages, use the non-destructive pmap_is_referenced() instead of the
destructive pmap_ts_referenced() and deactivate pages accordingly.

This function is racy before and after this change: a reference bit
which is set between the pmap_is_referenced() and
vm_page_try_remove_all() calls is lost. We do not have a pmap interface
for removing only unreferenced mappings.

[markj]

Re-add page locking which I removed prematurely.

[kib]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

Does it make sense to move these checks before tryxbusy() ? Same for pmap_is_referenced() call below ?

[markj]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

The wired check must come after the busy (the holder of a page busy is allowed to wire the page), but otherwise I think this is reasonable.

[kib]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

I suspect that the checks should be repeated after xbusy succeeded. At least exists_quick(), and then wired().

[jeff]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

What are you trying to save by reordering?

[kib]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

I want to execute tests that do not change the page' state before we modify the page (xbusy). I do think that in some situations the page_exists_quick() or wired() checking before xbusy would save a lot.

[jeff]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

page_exists_quick() is still much more expensive than busy. If we have to repeat it it seems like a loss to me. busy is only an atomic. For wired you could check before and after with little cost.

[markj]

sys/vm/vm_swapout.c

183 ↗

(On Diff #65275)

I think we could check wired and exists_quick() before busy, and repeat the wired check after. I think the exists_quick() check is probably too expensive to repeat.

[markj]

Check for wirings before busying the page.