memcg: Support per-memcg KSM metrics

From: xu xin <xu.xin16@zte.com.cn>

v2->v3:
------
Some fixes of compilation error due to missed inclusion of header or missed
function definition on some kernel config.
https://lore.kernel.org/all/202509142147.WQI0impC-lkp@int...
https://lore.kernel.org/all/202509142046.QatEaTQV-lkp@int...

v1->v2:
------
According to Shakeel's suggestion, expose these metric item into memory.stat
instead of a new interface.
https://lore.kernel.org/all/ir2s6sqi6hrbz7ghmfngbif6fbgms...

Background
==========

With the enablement of container-level KSM (e.g., via prctl [1]), there is
a growing demand for container-level observability of KSM behavior. However,
current cgroup implementations lack support for exposing KSM-related metrics.

So add the counter in the existing memory.stat without adding a new interface.
To diaplay per-memcg KSM statistic counters,  we traverse all processes of a
memcg and summing the processes' ksm_rmap_items counters instead of adding enum
item in memcg_stat_item or node_stat_item and updating the corresponding enum
counter when ksmd manipulate pages.

Now Linux users can look up all per-memcg KSM counters by:

# cat /sys/fs/cgroup/xuxin/memory.stat | grep ksm
ksm_rmap_items 0
ksm_zero_pages 0
ksm_merging_pages 0
ksm_profit 0

Q&A
====
why don't I add enum item in memcg_stat_item or node_stat_item like
other items in memory.stat ?

I tried the way of adding enum item in memcg_stat_item and updating them when
ksmd manipulate pages, but it failed with error statistic ksm counters of
memcg. This is because of the following reasons:

1) The KSM counter of memcgroup can be correctly incremented, but cannot be
properly decremented. E,g,, when ksmd scans pages of a process, it can use
the mm_struct of the struct ksm_rmap_item to reverse-lookup the memcg
and then increase the value via mod_memcg_state(memcg, MEMCG_KSM_COUNT, 1).
However, when the process exits abruptly, since ksmd asynchronously scans
the mmslot list in the background, it is no longer able to correctly locate
the original memcg through mm_struct by get_mem_cgroup_from_mm(), as the
task_struct has already been freed.

2) The first issue could potentially be addressed by adding a memcg
pointer directly into the ksm_rmap_item structure. However, this
increases memory overhead, especially when there are a large
number of ksm_rmap_items in the system (due to a high volume of
pages being scanned by ksmd). Moreover, this approach does not
resolve the same problem for ksm_zero_pages, because updates to
ksm_zero_pages are not performed through ksm_rmap_item, but
rather directly during unmap or page table entry (pte) faults
based on the mm_struct. At that point, if the process has
already exited, the corresponding memcg can no longer be
accurately identified.

xu xin (6):
  memcg: add per-memcg ksm_rmap_items stat
  memcg: show ksm_zero_pages count in memory.stat
  memcg: show ksm_merging_pages in memory.stat
  ksm: make ksm_process_profit available on CONFIG_PROCFS=n
  memcg: add per-memcg ksm_profit
  Documentation: add KSM statistic counters description in cgroup-v2.rst

 Documentation/admin-guide/cgroup-v2.rst | 17 ++++++
 include/linux/ksm.h                     |  1 +
 mm/ksm.c                                | 70 ++++++++++++++++++++++---
 mm/memcontrol.c                         |  5 ++
 4 files changed, 85 insertions(+), 8 deletions(-)

-- 
2.25.1