proc/ksm: add ksm stats to /proc/pid/smaps

With madvise and prctl KSM can be enabled for different VMA's.  Once it is
enabled we can query how effective KSM is overall.  However we cannot
easily query if an individual VMA benefits from KSM.

This commit adds a KSM section to the /prod/<pid>/smaps file.  It reports
how many of the pages are KSM pages.  Note that KSM-placed zeropages are
not included, only actual KSM pages.

Here is a typical output:

7f420a000000-7f421a000000 rw-p 00000000 00:00 0
Size:             262144 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:               51212 kB
Pss:                8276 kB
Shared_Clean:        172 kB
Shared_Dirty:      42996 kB
Private_Clean:       196 kB
Private_Dirty:      7848 kB
Referenced:        15388 kB
Anonymous:         51212 kB
KSM:               41376 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
FilePmdMapped:         0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:             202016 kB
SwapPss:            3882 kB
Locked:                0 kB
THPeligible:    0
ProtectionKey:         0
ksm_state:          0
ksm_skip_base:      0
ksm_skip_count:     0
VmFlags: rd wr mr mw me nr mg anon

This information also helps with the following workflow:
- First enable KSM for all the VMA's of a process with prctl.
- Then analyze with the above smaps report which VMA's benefit the most
- Change the application (if possible) to add the corresponding madvise
calls for the VMA's that benefit the most

[shr@devkernel.io: v5]
  Link: https://lkml.kernel.org/r/20230823170107.1457915-1-shr@devkernel.io
Link: https://lkml.kernel.org/r/20230822180539.1424843-1-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 7897a7dafcbc18..d49af173b5af91 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -461,6 +461,7 @@ Memory Area, or VMA) there is a series of lines such as the following::
     Private_Dirty:         0 kB
     Referenced:          892 kB
     Anonymous:             0 kB
+    KSM:                   0 kB
     LazyFree:              0 kB
     AnonHugePages:         0 kB
     ShmemPmdMapped:        0 kB
@@ -501,6 +502,9 @@ accessed.
 a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
 and a page is modified, the file page is replaced by a private anonymous copy.
 
+"KSM" reports how many of the pages are KSM pages. Note that KSM-placed zeropages
+are not included, only actual KSM pages.
+
 "LazyFree" shows the amount of memory which is marked by madvise(MADV_FREE).
 The memory isn't freed immediately with madvise(). It's freed in memory
 pressure if the memory is clean. Please note that the printed value might