sendfile and page usage statistics

[David G Lawrence]

> The read() from file certainly updates vnode pages' PG_A/PG_REFERENCED,
> because coping to userland is made by CPU. I think write() does the same.

   No, it does not. The kernel virtual mappings involved in that copy
are "special"; page bits (PG_A, PG_M, etc) from those mapping are not
preserved from the PTEs (at least not on x86).
 
-DG

David G. Lawrence
President
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