[PATCH v2] docs: proc.rst: meminfo: briefly describe gaps in memory accounting

[Mike Rapoport <rppt@kernel.org>]

From: Mike Rapoport <rppt@linux.ibm.com>

Add a paragraph that explains that it may happen that the counters in
/proc/meminfo do not add up to the overall memory usage.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
v2:
* Add brief changelog
* Fix typo
* Update example about network memory usage according to Eric's comment at

https://lore.kernel.org/lkml/CANn89iKprp7WYeZy4RRO5jHykprnSCcVBc7Tk14Ui_MA9OK7Fg@mail.gmail.com

v1: Link: https://lore.kernel.org/lkml/20210420085105.1156640-1-rppt@kernel.org

 Documentation/filesystems/proc.rst | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 48fbfc336ebf..8c77a491c436 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -929,8 +929,14 @@ meminfo
 ~~~~~~~
 
 Provides information about distribution and utilization of memory.  This
-varies by architecture and compile options.  The following is from a
-16GB PIII, which has highmem enabled.  You may not have all of these fields.
+varies by architecture and compile options. Please note that it may happen
+that the memory accounted here does not add up to the overall memory usage
+and the difference for some workloads can be substantial. In many cases there
+are other means to find out additional memory using subsystem specific
+interfaces, for instance /proc/net/sockstat for TCP memory allocations.
+
+The following is from a 16GB PIII, which has highmem enabled.
+You may not have all of these fields.
 
 ::
 
-- 
2.29.2