mm, THP, swap: delay splitting THP after swapped out

In this patch, splitting transparent huge page (THP) during swapping out
is delayed from after adding the THP into the swap cache to after
swapping out finishes.  After the patch, more operations for the
anonymous THP reclaiming, such as writing the THP to the swap device,
removing the THP from the swap cache could be batched.  So that the
performance of anonymous THP swapping out could be improved.

This is the second step for the THP swap support.  The plan is to delay
splitting the THP step by step and avoid splitting the THP finally.

With the patchset, the swap out throughput improves 42% (from about
5.81GB/s to about 8.25GB/s) in the vm-scalability swap-w-seq test case
with 16 processes.  At the same time, the IPI (reflect TLB flushing)
reduced about 78.9%.  The test is done on a Xeon E5 v3 system.  The swap
device used is a RAM simulated PMEM (persistent memory) device.  To test
the sequential swapping out, the test case creates 8 processes, which
sequentially allocate and write to the anonymous pages until the RAM and
part of the swap device is used up.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Ross Zwisler <[email protected]> [for brd.c, zram_drv.c, pmem.c]
Cc: Vishal L Verma <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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