mm/swap: select swap device with default priority round robin

mm/swap: select swap device with default priority round robin

Swap devices are assumed to have similar accessing speed when swapon if no
priority is specified.  It's unfair and doesn't make sense just because
one swap device is swapped on firstly, its priority will be higher than
the one swapped on later.

Here, set all swap devicess to have priority '-1' by default.  With this
change, swap device with default priority will be selected round robin
when swapping out.  This can improve the swapping efficiency a lot among
multiple swap devices with default priority.

Below are swapon output during the processes when high pressure
vm-scability test is being taken:

1) This is pre-commit a2468cc9bfdf, swap device is selectd one by one by
   priority from high to low when one swap device is exhausted:
------------------------------------
[root@hp-dl385g10-03 ~]# swapon
NAME       TYPE      SIZE   USED PRIO
/dev/zram0 partition  16G    16G   -1
/dev/zram1 partition  16G 966.2M   -2
/dev/zram2 partition  16G     0B   -3
/dev/zram3 partition  16G     0B   -4

2) This is behaviour with commit a2468cc9bfdf, on node, swap device
   sharing the same node id is selected firstly until exhausted; while
   on node no swap device sharing the node id it selects the one with
   highest priority until exhaustd:
------------------------------------
[root@hp-dl385g10-03 ~]# swapon
NAME       TYPE      SIZE  USED PRIO
/dev/zram0 partition  16G 15.7G   -2
/dev/zram1 partition  16G  3.4G   -3
/dev/zram2 partition  16G  3.4G   -4
/dev/zram3 partition  16G  2.6G   -5

3) After this patch applied, swap devices with default priority are selectd
   round robin:
------------------------------------
[root@hp-dl385g10-03 block]# swapon
NAME       TYPE      SIZE USED PRIO
/dev/zram0 partition  16G 6.6G   -1
/dev/zram1 partition  16G 6.6G   -1
/dev/zram2 partition  16G 6.6G   -1
/dev/zram3 partition  16G 6.6G   -1

With the change, about 18% efficiency promotion relative to node based
way as below. (Surely, the pre-commit a2468cc9bfdf way is the worst.)

vm-scability test:
==================
Test with:
usemem --init-time -O -y -x -n 31 2G (4G memcg, zram as swap)
                            one by one:      node based:      round robin:
System time:                1087.38 s        637.92 s         526.74 s     (lower is better)
Sum Throughput:             2036.55 MB/s     3546.56 MB/s     4207.56 MB/s (higher is better)
Single process Throughput:  65.69 MB/s       114.40 MB/s      135.72 MB/s  (high is better)
free latency:               15769409.48 us   10138455.99 us   6810119.01 us(lower is better)

Link: https://lkml.kernel.org/r/20251028034308.929550-3-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>