[PATCH] writeback scalability improvements

The kernel has a number of problems wrt heavy write traffic to multiple
spindles.  What keeps on happening is that all processes which are
responsible for writeback get blocked on one of the queues and all the
others fall idle.

This happens in the balance_dirty_pages() path (balance_dirty() in 2.4)
and in the page reclaim code, when a dirty page is found on the LRU.

The latter is particularly bad because it causes "innocent" processes
to be suspended for long periods due to the activity of heavy writers.

The general idea is: the primary resource for writeback should be the
process which is dirtying memory.  The secondary resource is the
pdflush pool (although this is mainly for providing async writeback in
the presence of light-moderate loads).  Add the final
oh-gee-we-screwed-up resource for writeback is a caller to
shrink_cache().

This patch addresses the balance_dirty_pages() path.  This code was
initially modelled on the 2.4 writeback scheme: throttled processes
writeback all data regardless of its queue.  Instead, the patch changes
it so that the balance_dirty_pages() caller only writes back pages
which are dirty against the queue which that caller just dirtied.

So the effect is a better allocation of writeback resources across the
queues and increased parallelism.

The per-queue writeback is implemented by using
mapping->backing_dev_info as a search key during the walk across the
superblocks and inodes.

The patch also fixes an initialisation problem in
block_dev.c:do_open(): it was setting up the blockdev's
mapping->backing_dev_info too early, before the queue has been
identified.

Generally, this patch doesn't help much, because of the stalls in the
page allocator.  I have a patch which mostly fixes that up, and taken
together the kernel is achieving almost platter speed against six
spindles, but only when the system has a small amount of memory.  More
work is needed there.

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