Documentation/admin-guide/mm/pagemap.rst: drop "Using pagemap to do something useful"

That example was added in 2008.  In 2015, we restricted access to the PFNs
in the pagemap to CAP_SYS_ADMIN, making that approach quite less usable.

It's 2024 now, and using that racy and low-lewel mechanism to calculate
the USS should not be considered a good example anymore.  /proc/$pid/smaps
and /proc/$pid/smaps_rollup can do a much better job without any of that
low-level handling.

Let's just drop that example.

Link: https://lkml.kernel.org/r/20240607122357.115423-7-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Oscar Salvador <osalvador@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/pagemap.rst

@@ -173,27 +173,6 @@ LRU related page flags
 The page-types tool in the tools/mm directory can be used to query the
 above flags.
 
-Using pagemap to do something useful
-====================================
-
-The general procedure for using pagemap to find out about a process' memory
-usage goes like this:
-
- 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are
-    mapped to what.
- 2. Select the maps you are interested in -- all of them, or a particular
-    library, or the stack or the heap, etc.
- 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine.
- 4. Read a u64 for each page from pagemap.
- 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``.  For each PFN you
-    just read, seek to that entry in the file, and read the data you want.
-
-For example, to find the "unique set size" (USS), which is the amount of
-memory that a process is using that is not shared with any other process,
-you can go through every map in the process, find the PFNs, look those up
-in kpagecount, and tally up the number of pages that are only referenced
-once.
-
 Exceptions for Shared Memory
 ============================