Improve superpage handling in pmap_advise()

[alc]

Summary

In pmap_advise(), when we encounter a superpage mapping, we first demote the mapping and then destroy one of the 4 KB page mappings so that there is a potential trigger for repromotion. Currently, we destroy the first 4 KB page mapping that falls within the (current) superpage mapping or the virtual address range {sva, eva). However, I have found empirically that destroying the last 4 KB mapping produces slightly better results, specifically, more promotions and fewer failed promotion attempts. Accordingly, this revision changes pmap_advise() to destroy the last 4 KB page mapping. It also replaces some nearby uses of boolean_t with bool.

[alc]

Add context.

[markj]

amd64/amd64/pmap.c

7510

Should this comment be amended to explain why we choose the last page in the range?

[alc]

amd64/amd64/pmap.c

7510

Yes, I will do so this evening.