linux-kernel - [PATCH v1 6/6] Documentation/admin-guide/mm/pagemap.rst: drop "Using pagemap to do something useful"

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Message-ID: <20240607122357.115423-7-david@redhat.com>
Date: Fri,  7 Jun 2024 14:23:57 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>
Subject: [PATCH v1 6/6] 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.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 Documentation/admin-guide/mm/pagemap.rst | 21 ---------------------
 1 file changed, 21 deletions(-)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index f5f065c67615d..f2817a8015962 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/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
 ============================
 
-- 
2.45.2