Alter the block device ->direct_access() API to work with the new
get_xip_mem() API (that requires both kaddr and pfn are returned).

Some architectures will not do the right thing in their virt_to_page() for use
by XIP (to translate from the kernel virtual address returned by
direct_access(), to a user mappable pfn in XIP's page fault handler.

However, we can't switch it to just return the pfn and not the kaddr, because
we have no good way to get a kva from a pfn, and XIP requires the kva for its
read(2) and write(2) handlers.  So we have to return both.
Signed-off-by: NJared Hulbert <jaredeh@gmail.com>
Signed-off-by: NNick Piggin <npiggin@suse.de>
Cc: Carsten Otte <cotte@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux-mm@kvack.org
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

While looking at the implementation of the Ram backed block device
driver, I stumbled across a write-only local variable, which makes
little sense, so I assume it should actually work like this:
Signed-off-by: NPetr Tesarik <ptesarik@suse.cz>
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Support direct_access XIP method with brd.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

This is a rewrite of the ramdisk block device driver.

The old one is really difficult because it effectively implements a block
device which serves data out of its own buffer cache.  It relies on the dirty
bit being set, to pin its backing store in cache, however there are non
trivial paths which can clear the dirty bit (eg.  try_to_free_buffers()),
which had recently lead to data corruption.  And in general it is completely
wrong for a block device driver to do this.

The new one is more like a regular block device driver.  It has no idea about
vm/vfs stuff.  It's backing store is similar to the buffer cache (a simple
radix-tree of pages), but it doesn't know anything about page cache (the pages
in the radix tree are not pagecache pages).

There is one slight downside -- direct block device access and filesystem
metadata access goes through an extra copy and gets stored in RAM twice.
However, this downside is only slight, because the real buffercache of the
device is now reclaimable (because we're not playing crazy games with it), so
under memory intensive situations, footprint should effectively be the same --
maybe even a slight advantage to the new driver because it can also reclaim
buffer heads.

The fact that it now goes through all the regular vm/fs paths makes it
much more useful for testing, too.

   text    data     bss     dec     hex filename
   2837     849     384    4070     fe6 drivers/block/rd.o
   3528     371      12    3911     f47 drivers/block/brd.o

Text is larger, but data and bss are smaller, making total size smaller.

A few other nice things about it:
- Similar structure and layout to the new loop device handlinag.
- Dynamic ramdisk creation.
- Runtime flexible buffer head size (because it is no longer part of the
  ramdisk code).
- Boot / load time flexible ramdisk size, which could easily be extended
  to a per-ramdisk runtime changeable size (eg. with an ioctl).
- Can use highmem for the backing store.

[akpm@linux-foundation.org: fix build]
[byron.bbradley@gmail.com: make rd_size non-static]
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NByron Bradley <byron.bbradley@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>