RDMA/core: Acquire and release mmap_sem on page range

Currently mmap_sem is read locked while pinning the memory.  In a
multi-threaded application of a process, holding mmap_sem lock creates
contention with other threads who might be either registering memory,
creating QPs or simply doing mmap() as such operations also require to
hold the mmap_sem write lock.

All such operation cannot make forward progress until one memory pin
operation is completed.  It becomes more worse if the memory is unpinned
and/or memory registration is large (in GB range).

Therefore, instead of holding mmap_sem for too long (for whole region
pinning), acquire and release the lock for every few pages.  For example
on x86 with 4K page size, acquire and release mmap_sem for every 2Mbytes
memory chunk.

This allows other competing threads to make progress who might wish to
hold mmap_sem for shorter duration.

When memory registration latency is measured using [1] for memory sizes
ranging from 4K to 48GB, <= 1% or 0.5% degradation is noticed. In many
runs no difference is seen other than run-to-run variance.

In other targeted tests of users with large memory, desired improvements
are seen due to reduced contention of mmap_sem.

[1] https://github.com/paravmellanox/rtool



$ rdma_resource_lat -c 1 -s 48G -a -u L -i 500 -A

It registers pinned memory from 4K to 48GB size with 500 iterations for
each memory size.

$ rdma_resource_lat -c 1 -s 12G -a -u L -i 500 -t 4

4 competing threads pin memory, each of 12GB size with 500 iterations.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>