Various changes to pmap_ts_referenced()

[alc]

Summary

Move PMAP_TS_REFERENCED_MAX out of the various pmap implementations and into vm/pmap.h, and describe what its purpose is. Eliminate the archaic "XXX" comment about its value. I don't believe that its exact value, e.g., 5 versus 6, matters.

Update the arm64 and riscv pmap implementations of pmap_ts_referenced() to opportunistically update the page's dirty field.

On amd64, use the PDE value already cached in a local variable rather than dereferencing a pointer again and again.

I'm primarily looking for feedback on the first point, especially the new comment describing PMAP_TS_REFERENCED_MAX.

[kib]

May be, it is better to move the comments from MD pmap.c into pmap.h ? I do not see much use in repeating the text for each architecture.

[markj]

vm/pmap.h

111

Perhaps it would be worth adding a kassert to this effect to the active queue scan.

[alc]

vm/pmap.h

111

Yes, I agree. However, armv4 and powerpc/aim{32,64} clear all of the bits. (mips has a practically non-functional reference bit emulation mechanism, but it will never trigger an overflow because it never returns a value greater than 1 from pmap_ts_referenced().)

[alc]

In D7836#162545, @kib wrote:

May be, it is better to move the comments from MD pmap.c into pmap.h ? I do not see much use in repeating the text for each architecture.

Yes, I seriously considered this, but we don't have any similar comments in vm/pmap.h. The hair that I'm splitting here is that we're defining PMAP_TS_REFERENCED_MAX in vm/pmap.h and describing it at the point of definition but pmap_ts_referenced() is defined in pmap.c. I think your proposal would make sense as a part of an overall deduplication that described all of the public pmap functions in vm/pmap.h.