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From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
To:     Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        linux-doc@vger.kernel.org (open list:DOCUMENTATION),
        linux-kernel@vger.kernel.org (open list),
        linux-fsdevel@vger.kernel.org (open list:PROC FILESYSTEM),
        linux-mm@kvack.org (open list:MEMORY MANAGEMENT),
        linux-kselftest@vger.kernel.org (open list:KERNEL SELFTEST FRAMEWORK)
Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        kernel@collabora.com
Subject: [PATCH v2 4/4] mm: add documentation of the new ioctl on pagemap
Date:   Thu, 25 Aug 2022 12:09:26 +0500
Message-Id: <20220825070926.2922471-5-usama.anjum@collabora.com>
In-Reply-To: <20220825070926.2922471-1-usama.anjum@collabora.com>
References: <20220825070926.2922471-1-usama.anjum@collabora.com>
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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