[Intel-gfx] [CI] drm/i915: Keep a small stash of preallocated WC pages

[Chris Wilson]

Quoting Chris Wilson (2017-08-22 18:38:28)
> We use WC pages for coherent writes into the ppGTT on !llc
> architectures. However, to create a WC page requires a stop_machine(),
> i.e. is very slow. To compensate we currently keep a per-vm cache of
> recently freed pages, but we still see the slow startup of new contexts.
> We can amoritize that cost slightly by allocating WC pages in small
> batches (PAGEVEC_SIZE == 14) and since creating a WC page implies a
> stop_machine() there is no penalty for keeping that stash global.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Reviewed-by: Matthew Auld <matthew.auld at intel.com>

And pushed. I didn't split out the new dependency on struct_mutex to a
different mutex in the end, a task for later!
-Chris