linux-kernel - [RFC V2 37/37] Add documentation for dmemfs

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Message-Id: <6a3a71f75dad1fa440677fc1bcdc170f178be1d8.1607332046.git.yuleixzhang@tencent.com>
Date:   Mon,  7 Dec 2020 19:31:30 +0800
From:   yulei.kernel@...il.com
To:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, naoya.horiguchi@....com,
        viro@...iv.linux.org.uk, pbonzini@...hat.com
Cc:     joao.m.martins@...cle.com, rdunlap@...radead.org,
        sean.j.christopherson@...el.com, xiaoguangrong.eric@...il.com,
        kernellwp@...il.com, lihaiwei.kernel@...il.com,
        Yulei Zhang <yuleixzhang@...cent.com>
Subject: [RFC V2 37/37] 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 | 58 ++++++++++++++++++++++++++++++++++++
 Documentation/filesystems/index.rst  |  1 +
 2 files 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 00000000..f13ed0c
--- /dev/null
+++ b/Documentation/filesystems/dmemfs.rst
@@ -0,0 +1,58 @@
+.. 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
+
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 98f59a8..23e944b 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -120,3 +120,4 @@ Documentation for filesystem implementations.
    xfs-delayed-logging-design
    xfs-self-describing-metadata
    zonefs
+   dmemfs
-- 
1.8.3.1