mm, swap: clean up plist removal and adding

When the swap device is full (inuse_pages == pages), it should be removed
from the allocation available plist.  If any slot is freed, the swap
device should be added back to the plist.  Additionally, during swapon or
swapoff, the swap device is forcefully added or removed.

Currently, the condition (inuse_pages == pages) is checked after every
counter update, then remove or add the device accordingly.  This is
serialized by si->lock.

This commit decouples it from the protection of si->lock and reworked
plist removal and adding, making it possible to get rid of the hard
dependency on si->lock in allocation path in later commits.

To achieve this, simply using another lock is not an optimal approach, as
the overhead is observable for a hot counter, and may cause complex
locking issues.  Thus, this commit manages to make it a lock-free atomic
operation, by embedding the plist state into the second highest bit of the
atomic counter.

Simply making the counter an atomic will not work, if the update and plist
status check are not performed atomically, we may miss an addition or
removal.  With the embedded info we can update the counter and check the
plist status with single atomic operations, and avoid any extra overheads:

If the counter is full (inuse_pages == pages) and the off-list bit is
unset, we attempt to remove it from the plist.  If the counter is not full
(inuse_pages != pages) and the off-list bit is set, we attempt to add it
to the plist.  Removing, adding and bit update is serialized with a lock,
which is a cold path.  Ordinary counter updates will be lock-free.

Link: https://lkml.kernel.org/r/20250113175732.48099-7-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chis Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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