[PATCH v5 6/6] doc: Update documentation for page_idle virtual address indexing

[Joel Fernandes (Google)]

This patch updates the documentation with the new page_idle tracking
feature which uses virtual address indexing.

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Reviewed-by: Sandeep Patil <sspatil@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 .../admin-guide/mm/idle_page_tracking.rst     | 43 ++++++++++++++++---
 1 file changed, 36 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/mm/idle_page_tracking.rst b/Documentation/admin-guide/mm/idle_page_tracking.rst
index df9394fb39c2..9eef32000f5e 100644
--- a/Documentation/admin-guide/mm/idle_page_tracking.rst
+++ b/Documentation/admin-guide/mm/idle_page_tracking.rst
@@ -19,10 +19,14 @@ It is enabled by CONFIG_IDLE_PAGE_TRACKING=y.
 
 User API
 ========
+There are 2 ways to access the idle page tracking API. One uses physical
+address indexing, another uses a simpler virtual address indexing scheme.
 
-The idle page tracking API is located at ``/sys/kernel/mm/page_idle``.
-Currently, it consists of the only read-write file,
-``/sys/kernel/mm/page_idle/bitmap``.
+Physical address indexing
+-------------------------
+The idle page tracking API for physical address indexing using page frame
+numbers (PFN) is located at ``/sys/kernel/mm/page_idle``.  Currently, it
+consists of the only read-write file, ``/sys/kernel/mm/page_idle/bitmap``.
 
 The file implements a bitmap where each bit corresponds to a memory page. The
 bitmap is represented by an array of 8-byte integers, and the page at PFN #i is
@@ -74,6 +78,31 @@ See :ref:`Documentation/admin-guide/mm/pagemap.rst <pagemap>` for more
 information about ``/proc/pid/pagemap``, ``/proc/kpageflags``, and
 ``/proc/kpagecgroup``.
 
+Virtual address indexing
+------------------------
+The idle page tracking API for virtual address indexing using virtual frame
+numbers (VFN) for a process ``<pid>`` is located at ``/proc/<pid>/page_idle``.
+It is a bitmap that follows the same semantics as
+``/sys/kernel/mm/page_idle/bitmap`` except that it uses virtual instead of
+physical frame numbers.
+
+This idle page tracking API does not deal with PFN so it does not require prior
+lookups of ``pagemap``. This is an advantage on some systems where looking up
+PFN is considered a security issue.  Also in some cases, this interface could
+be slightly more reliable to use than physical address indexing, since in
+physical address indexing, address space changes can occur between reading the
+``pagemap`` and reading the ``bitmap``, while in virtual address indexing, the
+process's ``mmap_sem`` is held for the duration of the access.
+
+To estimate the amount of pages that are not used by a workload one should:
+
+ 1. Mark all the workload's pages as idle by setting corresponding bits in
+    ``/proc/<pid>/page_idle``.
+
+ 2. Wait until the workload accesses its working set.
+
+ 3. Read ``/proc/<pid>/page_idle`` and count the number of bits set.
+
 .. _impl_details:
 
 Implementation Details
@@ -99,10 +128,10 @@ When a dirty page is written to swap or disk as a result of memory reclaim or
 exceeding the dirty memory limit, it is not marked referenced.
 
 The idle memory tracking feature adds a new page flag, the Idle flag. This flag
-is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` (see the
-:ref:`User API <user_api>`
-section), and cleared automatically whenever a page is referenced as defined
-above.
+is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` for physical
+addressing or by writing to ``/proc/<pid>/page_idle`` for virtual
+addressing (see the :ref:`User API <user_api>` section), and cleared
+automatically whenever a page is referenced as defined above.
 
 When a page is marked idle, the Accessed bit must be cleared in all PTEs it is
 mapped to, otherwise we will not be able to detect accesses to the page coming
-- 
2.22.0.770.g0f2c4a37fd-goog