mm: swap: simplify struct percpu_cluster

mm: swap: simplify struct percpu_cluster

struct percpu_cluster stores the index of cpu's current cluster and the
offset of the next entry that will be allocated for the cpu.  These two
pieces of information are redundant because the cluster index is just
(offset / SWAPFILE_CLUSTER).  The only reason for explicitly keeping the
cluster index is because the structure used for it also has a flag to
indicate "no cluster".  However this data structure also contains a spin
lock, which is never used in this context, as a side effect the code
copies the spinlock_t structure, which is questionable coding practice in
my view.

So let's clean this up and store only the next offset, and use a sentinal
value (SWAP_NEXT_INVALID) to indicate "no cluster".  SWAP_NEXT_INVALID is
chosen to be 0, because 0 will never be seen legitimately; The first page
in the swap file is the swap header, which is always marked bad to prevent
it from being allocated as an entry.  This also prevents the cluster to
which it belongs being marked free, so it will never appear on the free
list.

This change saves 16 bytes per cpu.  And given we are shortly going to
extend this mechanism to be per-cpu-AND-per-order, we will end up saving
16 * 9 = 144 bytes per cpu, which adds up if you have 256 cpus in the
system.

Link: https://lkml.kernel.org/r/20240408183946.2991168-4-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>