BACKPORT: FROMLIST: mm: multigenerational lru: mm_struct list

To scan PTEs for accessed pages, a mm_struct list is maintained for
each memcg. When multiple threads traverse the same memcg->mm_list,
each of them gets a unique mm_struct and therefore they can run
walk_page_range() concurrently to reach page tables of all processes
of this memcg.

And to skip page tables of processes that have been sleeping since the
last walk, the usage of mm_struct is also tracked between context
switches.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
(am from https://lore.kernel.org/patchwork/patch/1478359/)

BUG=b:123039911
TEST=Built

Change-Id: I25d9eda8c6bdc7c3653b9f210a159d6c247c81e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3132023


Tested-by: Yu Zhao <yuzhao@chromium.org>
Commit-Queue: Yu Zhao <yuzhao@chromium.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Alexandre Marciano Gimenez <raging@google.com>