mm: multi-gen LRU: debugfs interface

Add /sys/kernel/debug/lru_gen for working set estimation and proactive
reclaim.  These techniques are commonly used to optimize job scheduling
(bin packing) in data centers [1][2].

Compared with the page table-based approach and the PFN-based
approach, this lruvec-based approach has the following advantages:
1. It offers better choices because it is aware of memcgs, NUMA nodes,
   shared mappings and unmapped page cache.
2. It is more scalable because it is O(nr_hot_pages), whereas the
   PFN-based approach is O(nr_total_pages).

Add /sys/kernel/debug/lru_gen_full for debugging.

[1] https://dl.acm.org/doi/10.1145/3297858.3304053
[2] https://dl.acm.org/doi/10.1145/3503222.3507731

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yu Zhao <[email protected]>
Reviewed-by: Qi Zheng <[email protected]>
Acked-by: Brian Geffon <[email protected]>
Acked-by: Jan Alexander Steffens (heftig) <[email protected]>
Acked-by: Oleksandr Natalenko <[email protected]>
Acked-by: Steven Barrett <[email protected]>
Acked-by: Suleiman Souhlal <[email protected]>
Tested-by: Daniel Byrne <[email protected]>
Tested-by: Donald Carr <[email protected]>
Tested-by: Holger Hoffstätte <[email protected]>
Tested-by: Konstantin Kharlamov <[email protected]>
Tested-by: Shuang Zhai <[email protected]>
Tested-by: Sofia Trinh <[email protected]>
Tested-by: Vaibhav Jain <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michael Larabel <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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