chuq@chuq.com said:
>  (2) when we're reading the page from memory to write it to backing
> store.
>      this use also involves careful ordering of operations in
>      pmap_clear_modify(), so that any writes to the page via other
>      mappings which could end up in different cache lines will also
>      result in the page being marked modified again, so that we'll
>      be forced to write the page to backing store again at some point
>      in the future. 

Hmm, I'm not sure if this is relevant, but the arm32 pmap_protect() 
implementation does not flush the cache when a page is marked read-only.  
I discovered this to my cost when trying to optimize pmap_copy_page to 
minimize cache flushes.

R.