mm: multi-gen LRU: kill switch

mm: multi-gen LRU: kill switch

Add /sys/kernel/mm/lru_gen/enabled as a kill switch. Components that
can be disabled include:
  0x0001: the multi-gen LRU core
  0x0002: walking page table, when arch_has_hw_pte_young() returns
          true
  0x0004: clearing the accessed bit in non-leaf PMD entries, when
          CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG=y
  [yYnN]: apply to all the components above
E.g.,
  echo y >/sys/kernel/mm/lru_gen/enabled
  cat /sys/kernel/mm/lru_gen/enabled
  0x0007
  echo 5 >/sys/kernel/mm/lru_gen/enabled
  cat /sys/kernel/mm/lru_gen/enabled
  0x0005

NB: the page table walks happen on the scale of seconds under heavy memory
pressure, in which case the mmap_lock contention is a lesser concern,
compared with the LRU lock contention and the I/O congestion.  So far the
only well-known case of the mmap_lock contention happens on Android, due
to Scudo [1] which allocates several thousand VMAs for merely a few
hundred MBs.  The SPF and the Maple Tree also have provided their own
assessments [2][3].  However, if walking page tables does worsen the
mmap_lock contention, the kill switch can be used to disable it.  In this
case the multi-gen LRU will suffer a minor performance degradation, as
shown previously.

Clearing the accessed bit in non-leaf PMD entries can also be disabled,
since this behavior was not tested on x86 varieties other than Intel and
AMD.

[1] https://source.android.com/devices/tech/debug/scudo
[2] https://lore.kernel.org/r/20220128131006.67712-1-michel@lespinasse.org/
[3] https://lore.kernel.org/r/20220202024137.2516438-1-Liam.Howlett@oracle.com/

Link: https://lkml.kernel.org/r/20220407031525.2368067-11-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Acked-by: Brian Geffon <bgeffon@google.com>
Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Acked-by: Steven Barrett <steven@liquorix.net>
Acked-by: Suleiman Souhlal <suleiman@google.com>
Tested-by: Daniel Byrne <djbyrne@mtu.edu>
Tested-by: Donald Carr <d@chaos-reins.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Tested-by: Shuang Zhai <szhai2@cs.rochester.edu>
Tested-by: Sofia Trinh <sofia.trinh@edi.works>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>