> If you were paging _out_, then vm_pageout_page() will simply re-activate
> the page... it doesn't get paged out, but then, no one else can use it,
> either (which is the real harm, there...)
> 
> If you were paging _in_, I _think_ what happens is that you will either
> take a SEGV (because the page "doesn't exist"), or, if you were attempting
> to page in a page that was part of a shadow object, you could end up
> getting the page from an upstream object... which may be Bad.

Luckily, swap_alloc() isn't in the path when paging in something. It's called
only once when first paging out that something (or something else in the same
range of swap blocks that the swap pager allocates).

The only way a page in can fail is because of a failure in the swap device's
strategy routine.

-pk