mm/swap: allocate swap slots in batches

Currently, the swap slots are allocated one page at a time, causing
contention to the swap_info lock protecting the swap partition on every
page being swapped.

This patch adds new functions get_swap_pages and scan_swap_map_slots to
request multiple swap slots at once.  This will reduces the lock
contention on the swap_info lock.  Also scan_swap_map_slots can operate
more efficiently as swap slots often occurs in clusters close to each
other on a swap device and it is quicker to allocate them together.

Link: http://lkml.kernel.org/r/9fec2845544371f62c3763d43510045e33d286a6.1484082593.git.tim.c.chen@linux.intel.com
Signed-off-by: Tim Chen <[email protected]>
Signed-off-by: "Huang, Ying" <[email protected]>
Cc: Aaron Lu <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]> escreveu:
Cc: Kirill A. Shutemov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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