pmap_copy: Clear the accessed bit when copying a managed superpage mapping

[alc]

Summary

pmap_copy() is used to speculatively create mappings, so those mappings should not have their access bit preset.

[alc]

The demotion code does not require superpage mappings to have their accessed bit set, so there is no reason not to clear it in pmap_copy().

[kib]

In D31162#701247, @alc wrote:

The demotion code does not require superpage mappings to have their accessed bit set, so there is no reason not to clear it in pmap_copy().

Please add this sentence to the comment.

[kib]

sys/amd64/amd64/pmap.c

7792

Why do you limit clearing to PG_MANAGED mappings?

[alc]

Update comments.

[alc]

sys/amd64/amd64/pmap.c

7792

We never actually test the accessed bit state on unmanaged mappings. We don't really care whether it represents the page having been accessed. So, typically, we preset the accessed bit on unmanaged mappings to eliminate the future cost of atomically setting it. Since most arm64 cores still use a software exception handler to set the accessed bit, presetting the accessed bit for unmanaged mappings arguably makes more sense on arm64 than it ever did on x86.