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Subject: Re: [RFC V2 37/37] Add documentation for dmemfs
To:     yulei.kernel@gmail.com, linux-mm@kvack.org,
        akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        naoya.horiguchi@nec.com, viro@zeniv.linux.org.uk,
        pbonzini@redhat.com
Cc:     joao.m.martins@oracle.com, sean.j.christopherson@intel.com,
        xiaoguangrong.eric@gmail.com, kernellwp@gmail.com,
        lihaiwei.kernel@gmail.com, Yulei Zhang <yuleixzhang@tencent.com>
References: <cover.1607332046.git.yuleixzhang@tencent.com>
 <6a3a71f75dad1fa440677fc1bcdc170f178be1d8.1607332046.git.yuleixzhang@tencent.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <a8920d25-e109-cfce-5137-1f4374c815e9@infradead.org>
Date:   Thu, 24 Dec 2020 10:27:28 -0800
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Hi,

On 12/7/20 3:31 AM, yulei.kernel@gmail.com wrote:
> From: Yulei Zhang <yuleixzhang@tencent.com>
> 
> Introduce dmemfs.rst to document the basic usage of dmemfs.
> 
> Signed-off-by: Yulei Zhang <yuleixzhang@tencent.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,

                                                             usages,
or                                                           uses,

> +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

                               boots up. The detail

> +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

Would anyone want DMEM_FS without 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

                                                     sizes

> +the userspace, for example ::

       userspace. For example::

> +
> +    # mount -t dmemfs none -o pagesize=4K /mnt/
> +
> +The it can create the backing storage with 4G size ::

   Then

> +
> +    # truncate /mnt/dmemfs-uuid --size 4G
> +
> +To use as backing storage for virtual machine starts with qemu, just need

                                                 started with qemu, just specify
   the memory-backed-file

> +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 \

                        backed


> +        share=yes,size=4G,host-nodes=0,policy=preferred -numa node,nodeid=0,memdev=ram-node0
> +


-- 
~Randy