mm/mglru: restructure the reclaim loop

The current loop will calculate the scan number on each iteration.  The
number of folios to scan is based on the LRU length, with some unclear
behaviors, e.g, the scan number is only shifted by reclaim priority when
aging is not needed or when at the default priority, and it couples the
number calculation with aging and rotation.

Adjust, simplify it, and decouple aging and rotation.  Just calculate the
scan number for once at the beginning of the reclaim, always respect the
reclaim priority, and make the aging and rotation more explicit.

This slightly changes how aging and offline memcg reclaim works:
Previously, aging was skipped at DEF_PRIORITY even when eviction was no
longer possible, so the reclaimer wasted an iteration until the priority
escalated.  Now aging runs immediately whenever it is needed to make
progress; the DEF_PRIORITY skip only applies when eviction is still
viable.  This may avoid wasted iterations that over-reclaim slab and break
reclaim balance in multi-cgroup setups.

Similar for offline memcg.  Previously, offline memcg wouldn't be aged
unless it didn't have any evictable folios.  Now, we might age it if it
has only 3 generations, which should be fine.  On one hand, offline memcg
might still hold long-term folios, and in fact, a long-existing offline
memcg must be pinned by some long-term folios like shmem.  These folios
might be used by other memcg, so aging them as ordinary memcg seems
correct.  Besides, aging enables further reclaim of an offlined memcg,
which will certainly happen if we keep shrinking it.  And offline memcg
might soon be no longer an issue with reparenting.

Overall, the memcg LRU rotation, as described in mmzone.h, remains the
same.

Note that because the scan budget is now pinned at loop entry, tiny lruvec
might skip this reclaim pass, also skipping aging, which could be
beneficial as aging is not helpful since it will still be un-reclaimable
after aging.  Reclaim will go on as usual once priority escalates.

Link: https://lore.kernel.org/20260428-mglru-reclaim-v7-4-02fabb92dc43@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chen Ridong <chenridong@huaweicloud.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Stevens <stevensd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Leno Hou <lenohou@gmail.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yafang <laoar.shao@gmail.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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