* 2.1.88, adds a bunch of new functionality to
  the swapper.  The main changes are:

* All swapping goes through the swap cache (aka. page cache) now.

* There is no longer a swap lock map.  Because we need to atomically
  test and create a new swap-cache page in order to do swap IO, it is
  sufficient just to lock the struct page itself.  Having only one
  layer of locking to deal with removes a number of races concerning
  swapping shared pages.

* We can swap shared pages, and still keep them shared when they are
  swapped back in!!!  Currently, only private shared pages (as in pages
  shared after a fork()) benefit from this, but the basic mechanism will
  be appropriate for MAP_ANONYMOUS | MAP_SHARED pages too
  (implementation to follow).  Pages will remain shared after a swapoff.

* The page cache is now quite happy dealing with swap-cache pages too.
  In particular, write-ahead and read-ahead of swap through the page
  cache will work fine (and in fact, write-ahead does get done already
  under certain circumstances with this patch --- that's essentially how
  the swapping of shared pages gets done).  Support code to perform
  asynchronous readahead of swap is included, but is not actually used
  anywhere yet.

  I've tested with a number of forked processes running with a shared
  working set larger than physical memory, and with SysV shared memory.
  I haven't found any problems with it so far.

Linus: I've also changed the way we consider us to need more memory in kswapd,
       but that was entirely orthogonal and did not impact these patches. ]

[Changelog pieced together by davej]