In databases it is common to have multiple threads or processes performing
O_SYNC writes against different parts of the same file.

Our performance at this is poor, because each writer blocks access to the
file by waiting on I/O completion while holding i_sem: everything is
serialised.

The patch improves things by moving the writing and waiting outside i_sem.
So other threads can get in and submit their I/O and permit the disk
scheduler to optimise the IO patterns better.

Also, the O_SYNC writer only writes and waits on the pages which he wrote,
rather than writing and waiting on all dirty pages in the file.

The reason we haven't been able to do this before is that the required walk
of the address_space page lists is easily livelockable without the i_sem
serialisation.  But in this patch we perform the waiting via a radix-tree
walk of the affected pages.  This cannot be livelocked.

The sync of the inode's metadata is still performed inside i_sem.  This is
because it is list-based and is hence still livelockable.  However it is
usually the case that databases are overwriting existing file blocks and
there will be no dirty buffers attached to the address_space anyway.

The code is careful to ensure that the IO for the pages and the IO for the
metadata are nonblockingly scheduled at the same time.  This is am improvemtn
over the current code, which will issue two separate write-and-wait cycles:
one for metadata, one for pages.

Note from Suparna:
Reworked to use the tagged radix-tree based writeback infrastructure.
Signed-off-by: Suparna Bhattacharya <suparna@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>