Now swap cluster allocator arranges the clusters in LRU style, so the
"cold" cluster stay at the head of nonfull lists are the ones that were
used for allocation long time ago and still partially occupied.  So if
allocator can't find enough contiguous slots to satisfy an high order
allocation, it's unlikely there will be slot being free on them to satisfy
the allocation, at least in a short period.

As a result, nonfull cluster scanning will waste time repeatly scanning
the unusable head of the list.

Also, multiple CPUs could content on the same head cluster of nonfull
list.  Unlike free clusters which are removed from the list when any CPU
starts using it, nonfull cluster stays on the head.

So introduce a new list frag list, all scanned nonfull clusters will be
moved to this list.  Both for avoiding repeated scanning and contention.

Frag list is still used as fallback for allocations, so if one CPU failed
to allocate one order of slots, it can still steal other CPU's clusters. 
And order 0 will favor the fragmented clusters to better protect nonfull
clusters

If any slots on a fragment list are being freed, move the fragment list
back to nonfull list indicating it worth another scan on the cluster. 
Compared to scan upon freeing a slot, this keep the scanning lazy and save
some CPU if there are still other clusters to use.

It may seems unneccessay to keep the fragmented cluster on list at all if
they can't be used for specific order allocation.  But this will start to
make sense once reclaim dring scanning is ready.

Link: https://lkml.kernel.org/r/20240730-swap-allocator-v5-7-cb9c148b9297@kernel.orgSigned-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>