This would fix the following failure during GC:

 nilfs_cpfile_delete_checkpoints: cannot delete block
 NILFS: GC failed during preparation: cannot delete checkpoints: err=-2

The problem was caused by a break in state consistency between page
cache and btree; the above block was removed from the btree but the
page buffering the block was remaining in the page cache in dirty
state.

This resolves the inconsistency by ensuring to clear dirty state of
the page buffering the deleted block.
Reported-by: NDavid Arendt <admin@prnet.org>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

This fixes the following circular locking dependency problem:

 =======================================================
 [ INFO: possible circular locking dependency detected ]
 2.6.30-rc3 #5
 -------------------------------------------------------
 segctord/3895 is trying to acquire lock:
  (&nilfs->ns_writer_mutex){+.+...}, at: [<d0d02172>]
   nilfs_mdt_get_block+0x89/0x20f [nilfs2]

 but task is already holding lock:
  (&bmap->b_sem){++++..}, at: [<d0d02d99>]
   nilfs_bmap_propagate+0x14/0x2e [nilfs2]

 which lock already depends on the new lock.

The bugfix is done by replacing call sites of nilfs_get_writer() which
are never called from read-only context with direct dereferencing of
pointer to a writable FS-instance.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

Pekka Enberg advised me:
> It would be nice if BUG(), BUG_ON(), and panic() calls would be
> converted to proper error handling using WARN_ON() calls. The BUG()
> call in nilfs_cpfile_delete_checkpoints(), for example, looks to be
> triggerable from user-space via the ioctl() system call.

This will follow the comment and keep them to a minimum.
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Pekka Enberg pointed out that double error handlings found after
nilfs_transaction_end() can be avoided by separating abort operation:

 OK, I don't understand this. The only way nilfs_transaction_end() can
 fail is if we have NILFS_TI_SYNC set and we fail to construct the
 segment. But why do we want to construct a segment if we don't commit?

 I guess what I'm asking is why don't we have a separate
 nilfs_transaction_abort() function that can't fail for the erroneous
 case to avoid this double error value tracking thing?

This does the separation and renames nilfs_transaction_end() to
nilfs_transaction_commit() for clarification.

Since, some calls of these functions were used just for exclusion control
against the segment constructor, they are replaced with semaphore
operations.
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

This adds the meta data file, which serves common buffer functions to the
DAT, sufile, cpfile, ifile, and so forth.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>