BACKPORT: FROMLIST: mm: multigenerational lru: protection

The protection is based on page access types and patterns. There are
two access types: one via page tables and the other via file
descriptors. The protection of the former type is by design stronger
because:
  1) The uncertainty in determining the access patterns of the former
  type is higher due to the coalesced nature of the accessed bit.
  2) The cost of evicting the former type is higher due to the TLB
  flushes required and the likelihood of involving I/O.
  3) The penalty of under-protecting the former type is higher because
  applications usually do not prepare themselves for major faults like
  they do for blocked I/O. For example, client applications commonly
  dedicate blocked I/O to separate threads to avoid UI janks that
  negatively affect user experience.

There are also two access patterns: one with temporal locality and the
other without. The latter pattern, e.g., random and sequential, needs
to be explicitly excluded to avoid weakening the protection of the
former pattern. Generally the former type follows the former pattern
unless MADV_SEQUENTIAL is specified and the latter type follows the
latter pattern unless outlying refaults have been observed.

Upon faulting, a page is added to the youngest generation, which
provides the strongest protection as the eviction will not consider
this page before the aging has scanned it at least twice. The first
scan clears the accessed bit set during the initial fault. And the
second scan makes sure this page has not been used since the first
scan. A page from any other generations is brought back to the
youngest generation whenever the aging finds the accessed bit set on
any of the PTEs mapping this page.

Unmapped pages are initially added to the oldest generation and then
conditionally protected by tiers. Pages accessed N times via file
descriptors belong to tier order_base_2(N). Each tier keeps track of
how many pages from it have refaulted. Tier 0 is the base tier and
pages from it are evicted unconditionally because there are no better
candidates. Pages from an upper tier are either evicted or moved to
the next generation, depending on whether this upper tier has a higher
refault rate than the base tier. This model has the following
advantages:
  1) It removes the cost in the buffered access path and reduces the
  overall cost of protection because pages are conditionally protected
  in the reclaim path.
  2) It takes mapped pages into account and avoids overprotecting
  pages accessed multiple times via file descriptors.
  3 Additional tiers improve the protection of pages accessed more
  than twice.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
(am from https://lore.kernel.org/patchwork/patch/1478358/)

BUG=b:123039911
TEST=Built

Change-Id: Ibc9c90757fd095cdcc0a49823ada6b55f17ffc06
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3132022


Tested-by: Yu Zhao <yuzhao@chromium.org>
Commit-Queue: Yu Zhao <yuzhao@chromium.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>