[ I reversed the order in which writeback walks the superblock's
  dirty inodes.  It sped up dbench's unlink phase greatly.  I'm
  such a sleaze ]

The core writeback patch.  Switches file writeback from the dirty
buffer LRU over to address_space.dirty_pages.

- The buffer LRU is removed

- The buffer hash is removed (uses blockdev pagecache lookups)

- The bdflush and kupdate functions are implemented against
  address_spaces, via pdflush.

- The relationship between pages and buffers is changed.

  - If a page has dirty buffers, it is marked dirty
  - If a page is marked dirty, it *may* have dirty buffers.
  - A dirty page may be "partially dirty".  block_write_full_page
    discovers this.

- A bunch of consistency checks of the form

	if (!something_which_should_be_true())
		buffer_error();

  have been introduced.  These fog the code up but are important for
  ensuring that the new buffer/page code is working correctly.

- New locking (inode.i_bufferlist_lock) is introduced for exclusion
  from try_to_free_buffers().  This is needed because set_page_dirty
  is called under spinlock, so it cannot lock the page.  But it
  needs access to page->buffers to set them all dirty.

  i_bufferlist_lock is also used to protect inode.i_dirty_buffers.

- fs/inode.c has been split: all the code related to file data writeback
  has been moved into fs/fs-writeback.c

- Code related to file data writeback at the address_space level is in
  the new mm/page-writeback.c

- try_to_free_buffers() is now non-blocking

- Switches vmscan.c over to understand that all pages with dirty data
  are now marked dirty.

- Introduces a new a_op for VM writeback:

	->vm_writeback(struct page *page, int *nr_to_write)

  this is a bit half-baked at present.  The intent is that the address_space
  is given the opportunity to perform clustered writeback.  To allow it to
  opportunistically write out disk-contiguous dirty data which may be in other zones.
  To allow delayed-allocate filesystems to get good disk layout.

- Added address_space.io_pages.  Pages which are being prepared for
  writeback.  This is here for two reasons:

  1: It will be needed later, when BIOs are assembled direct
     against pagecache, bypassing the buffer layer.  It avoids a
     deadlock which would occur if someone moved the page back onto the
     dirty_pages list after it was added to the BIO, but before it was
     submitted.  (hmm.  This may not be a problem with PG_writeback logic).

  2: Avoids a livelock which would occur if some other thread is continually
     redirtying pages.

- There are two known performance problems in this code:

  1: Pages which are locked for writeback cause undesirable
     blocking when they are being overwritten.  A patch which leaves
     pages unlocked during writeback comes later in the series.

  2: While inodes are under writeback, they are locked.  This
     causes namespace lookups against the file to get unnecessarily
     blocked in wait_on_inode().  This is a fairly minor problem.

     I don't have a fix for this at present - I'll fix this when I
     attach dirty address_spaces direct to super_blocks.

- The patch vastly increases the amount of dirty data which the
  kernel permits highmem machines to maintain.  This is because the
  balancing decisions are made against the amount of memory in the
  machine, not against the amount of buffercache-allocatable memory.

  This may be very wrong, although it works fine for me (2.5 gigs).

  We can trivially go back to the old-style throttling with
  s/nr_free_pagecache_pages/nr_free_buffer_pages/ in
  balance_dirty_pages().  But better would be to allow blockdev
  mappings to use highmem (I'm thinking about this one, slowly).  And
  to move writer-throttling and writeback decisions into the VM (modulo
  the file-overwriting problem).

- Drops 24 bytes from struct buffer_head.  More to come.

- There's some gunk like super_block.flags:MS_FLUSHING which needs to
  be killed.  Need a better way of providing collision avoidance
  between pdflush threads, to prevent more than one pdflush thread
  working a disk at the same time.

  The correct way to do that is to put a flag in the request queue to
  say "there's a pdlfush thread working this disk".  This is easy to
  do: just generalise the "ra_pages" pointer to point at a struct which
  includes ra_pages and the new collision-avoidance flag.