mm: swap: allow cache reclaim to skip slot cache

mm: swap: allow cache reclaim to skip slot cache

Currently we free the reclaimed slots through slot cache even if the slot
is required to be empty immediately.  As a result the reclaim caller will
see the slot still occupied even after a successful reclaim, and need to
keep reclaiming until slot cache get flushed.  This caused ineffective or
over reclaim when SWAP is under stress.

So introduce a new flag allowing the slot to be emptied bypassing the slot
cache.

[21cnbao@gmail.com: small folios should have nr_pages == 1 but not nr_page == 0]
Link: https://lkml.kernel.org/r/20240805015324.45134-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20240730-swap-allocator-v5-6-cb9c148b9297@kernel.org
Signed-off-by: Kairui Song <kasong@tencent.com>
Reported-by: Barry Song <21cnbao@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>