Do not loose dirty bits for removing PROT_WRITE on arm64.

[kib]

Summary

Arm64 pmap interprets accessed writable ptes as modified, since ARMv8.0 does not track Dirty Bit Modifier in hardware. If writable bit is removed, page must be marked as dirty for MI VM.

This change is most important for COW, where fork caused losing content of the dirty pages which were not yet scanned by pagedaemon.

[alc]

pmap_remove() needs the same fix.

[alc]

P.S. Please change "loose" to "lose" when you commit this revision. :-)

[kib]

In D10339#214208, @alc wrote:

pmap_remove() needs the same fix.

Could you, please, elaborate ? pmap_remove() calls pmap_remove_l3() to clear pte, and the call to vm_page_dirty() is there. Superpages are always demoted before removal.

[alc]

I am wrong about pmap_remove(). You are correct. It does correctly update each page's dirty field.

[emaste]

Committed as rS316679