vm: add unlocked page lookup before trying vm_fault_soft_fast

[mjg]

Summary

Shaves a read lock + tryupgrade trip most of the time.

Stats from doing a kernel build (counters not present in the tree):
vm.fault_soft_fast_ok: 262653
vm.fault_soft_fast_failed_other: 41
vm.fault_soft_fast_failed_no_page: 39595772
vm.fault_soft_fast_failed_page_busy: 1929
vm.fault_soft_fast_failed_page_invalid: 22183

[kib]

sys/vm/vm_fault.c

365–366

There is no reason to busy the object if the page lookup validation fails, since you check for the busy page status in separate statement.

366–374

There should be a blank line before the multiline comment

372

And the blank line after the block of code described by the multiline comment

[mjg]

sys/vm/vm_fault.c

365–366

you mean something like this?

if (m->object != fs->first_object || m->pindex != fs->first_pindex)
        goto fail;

vm_object_busy(fs->first_object);

if (!vm_page_all_valid(m) ||
    ((fs->prot & VM_PROT_WRITE) != 0 && vm_page_busied(m)))
        goto fail_busy;

[kib]

sys/vm/vm_fault.c

365–366

Yes