mm: vmalloc: offload free_vmap_area_lock lock

Concurrent access to a global vmap space is a bottle-neck.  We can
simulate a high contention by running a vmalloc test suite.

To address it, introduce an effective vmap node logic.  Each node behaves
as independent entity.  When a node is accessed it serves a request
directly(if possible) from its pool.

This model has a size based pool for requests, i.e.  pools are serialized
and populated based on object size and real demand.  A maximum object size
that pool can handle is set to 256 pages.

This technique reduces a pressure on the global vmap lock.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Joel Fernandes (Google) <[email protected]>
Cc: Kazuhito Hagio <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Oleksiy Avramchenko <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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