page_launder() on 2.4.9/10 issue

[Marcelo Tosatti]

Linus,

I just noticed that the new page_launder() logic has a big bad problem.

The window to find and free previously written out pages by page_launder()
is the amount of writeable pages on the inactive dirty list.

We'll keep writing out dirty pages (as long as they are available) even if
have a ton of cleaned pages: its just that we don't see them because we
scan a small piece of the inactive dirty list each time.

That obviously did not happen with the full scan behaviour.

With asynchronous i_dirty->i_clean movement (moving a cleaned page to the
clean list at the IO completion handler. Please don't consider that for
2.4 :)) this would not happen, too.