On Fri, 2012-10-19 at 11:53 -0400, Rik van Riel wrote:
> 
> If we do need the extra refcount, why is normal
> page migration safe? :) 

Its mostly a matter of how convoluted you make the code, regular page
migration is about as bad as you can get

Normal does:

  follow_page(FOLL_GET) +1

  isolate_lru_page() +1

  put_page() -1

ending up with a page with a single reference (for anon, or one extra
each for the mapping and buffer).

And while I suppose I could do a put_page() in migrate_misplaced_page()
that makes the function frob the page-count depending on the return
value.

I always try and avoid conditional locks/refs, therefore the code ends
up doing:

  page = vm_normal_page()
  if (page) {
    get_page()

    migrate_misplaced_page()

    put_page()
  }


where migrate_misplaced_page() does isolate_lru_page()/putback_lru_page,
and this leaves the page-count invariant.

We got a ref, therefore we must put a ref, is easier than we got a ref
and must put except when...