[PATCH v3 4/4] mm: add documentation of the new ioctl on pagemap

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Add the explanation of the added ioctl on pagemap file. It can be used
to get, clear or both soft-dirty PTE bit of the specified range.
or both at the same time.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
Changes in v2:
- Update documentation to mention ioctl instead of the syscall
---
 Documentation/admin-guide/mm/soft-dirty.rst | 42 ++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/soft-dirty.rst b/Documentation/admin-guide/mm/soft-dirty.rst
index cb0cfd6672fa..d3d33e63a965 100644
--- a/Documentation/admin-guide/mm/soft-dirty.rst
+++ b/Documentation/admin-guide/mm/soft-dirty.rst
@@ -5,7 +5,12 @@ Soft-Dirty PTEs
 ===============
 
 The soft-dirty is a bit on a PTE which helps to track which pages a task
-writes to. In order to do this tracking one should
+writes to.
+
+Using Proc FS
+-------------
+
+In order to do this tracking one should
 
   1. Clear soft-dirty bits from the task's PTEs.
 
@@ -20,6 +25,41 @@ writes to. In order to do this tracking one should
      64-bit qword is the soft-dirty one. If set, the respective PTE was
      written to since step 1.
 
+Using IOCTL
+-----------
+
+The IOCTL on the ``/proc/PID/pagemap`` can be can be used to find the dirty pages
+atomically. The following commands are supported::
+
+	MEMWATCH_SD_GET
+		Get the page offsets which are soft dirty.
+
+	MEMWATCH_SD_CLEAR
+		Clear the pages which are soft dirty.
+
+	MEMWATCH_SD_GET_AND_CLEAR
+		Get and clear the pages which are soft dirty.
+
+The struct :c:type:`pagemap_sd_args` is used as the argument. In this struct:
+
+  1. The range is specified through start and len. The len argument need not be
+     the multiple of the page size, but since the information is returned for the
+     whole pages, len is effectively rounded up to the next multiple of the page
+     size.
+
+  2. The output buffer and size is specified in vec and vec_len. The offsets of
+     the dirty pages from start are returned in vec. The ioctl returns when the
+     whole range has been searched or vec is completely filled. The whole range
+     isn't cleared if vec fills up completely.
+
+  3. The flags can be specified in flags field. Currently only one flag,
+     PAGEMAP_SD_NO_REUSED_REGIONS is supported which can be specified to ignore
+     the VMA dirty flags for better performance. This flag shows only those pages
+     dirty which have been written to by the user. All new allocations aren't returned
+     to be dirty.
+
+Explanation
+-----------
 
 Internally, to do this tracking, the writable bit is cleared from PTEs
 when the soft-dirty bit is cleared. So, after this, when the task tries to
-- 
2.30.2