mm/mglru: rework aging feedback

mm/mglru: rework aging feedback

The aging feedback is based on both the number of generations and the
distribution of folios in each generation.  The number of generations is
currently the distance between max_seq and anon min_seq.  This is because
anon min_seq is not allowed to move past file min_seq.  The rationale for
that is that file is always evictable whereas anon is not.  However, for
use cases where anon is a lot cheaper than file:

1. Anon in the second oldest generation can be a better choice than
   file in the oldest generation.

2. A large amount of file in the oldest generation can skew the
   distribution, making should_run_aging() return false negative.

Allow anon and file min_seq to move independently, and use solely the
number of generations as the feedback for aging.  Specifically, when both
anon and file are evictable, anon min_seq can now be greater than file
min_seq, and therefore the number of generations becomes the distance
between max_seq and min(min_seq[0],min_seq[1]).  And should_run_aging()
returns true if and only if the number of generations is less than
MAX_NR_GENS.

As the first step to the final optimization, this change by itself should
not have userspace-visiable effects beyond performance.  The next twos
patch will take advantage of this change; the last patch in this series
will better distribute folios across MAX_NR_GENS.

[yuzhao@google.com: restore behaviour for systems with swappiness == 200]
Link: https://lkml.kernel.org/r/Z4S3-aJy5dj9tBTk@google.com
Link: https://lkml.kernel.org/r/20241231043538.4075764-4-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: David Stevens <stevensd@chromium.org>
Tested-by: Kalesh Singh <kaleshsingh@google.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>