linux-kernel - [PATCH 35/35] Add documentation for dmemfs

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Date:   Thu,  8 Oct 2020 15:54:25 +0800
From:   yulei.kernel@...il.com
To:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        viro@...iv.linux.org.uk, pbonzini@...hat.com
Cc:     linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, xiaoguangrong.eric@...il.com,
        kernellwp@...il.com, lihaiwei.kernel@...il.com,
        Yulei Zhang <yuleixzhang@...cent.com>
Subject: [PATCH 35/35] Add documentation for dmemfs

From: Yulei Zhang <yuleixzhang@...cent.com>

Introduce dmemfs.rst to document the basic usage of dmemfs.

Signed-off-by: Yulei Zhang <yuleixzhang@...cent.com>
---
 Documentation/filesystems/dmemfs.rst | 59 ++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 Documentation/filesystems/dmemfs.rst

diff --git a/Documentation/filesystems/dmemfs.rst b/Documentation/filesystems/dmemfs.rst
new file mode 100644
index 000000000000..cbb4cc1ed31d
--- /dev/null
+++ b/Documentation/filesystems/dmemfs.rst
@@ -0,0 +1,57 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+The Direct Memory Filesystem - DMEMFS
+=====================================
+
+
+.. Table of contents
+
+   - Overview
+   - Compilation
+   - Usage
+
+Overview
+========
+
+Dmemfs (Direct Memory filesystem) is device memory or reserved
+memory based filesystem. This kind of memory is special as it
+is not managed by kernel and it is without 'struct page'. Therefore
+it can save extra memory from the host system for various usage,
+especially for guest virtual machines.
+
+It uses a kernel boot parameter ``dmem=`` to reserve the system
+memory when the host system boots up, the details can be checked
+in /Documentation/admin-guide/kernel-parameters.txt.
+
+Compilation
+===========
+
+The filesystem should be enabled by turning on the kernel configuration
+options::
+
+        CONFIG_DMEM_FS          - Direct Memory filesystem support
+        CONFIG_DMEM             - Allow reservation of memory for dmem
+
+
+Additionally, the following can be turned on to aid debugging::
+
+        CONFIG_DMEM_DEBUG_FS    - Enable debug information for dmem
+
+Usage
+========
+
+Dmemfs supports mapping ``4K``, ``2M`` and ``1G`` size of pages to
+the userspace, for example ::
+
+    # mount -t dmemfs none -o pagesize=4K /mnt/
+
+The it can create the backing storage with 4G size ::
+
+    # truncate /mnt/dmemfs-uuid --size 4G
+
+To use as backing storage for virtual machine starts with qemu, just need
+to specify the memory-backed-file in the qemu command line like this ::
+
+    # -object memory-backend-file,id=ram-node0,mem-path=/mnt/dmemfs-uuid \
+        share=yes,size=4G,host-nodes=0,policy=preferred -numa node,nodeid=0,memdev=ram-node0
-- 
2.28.0