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

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Message-ID: <7f4f18b4-130e-ce51-3149-8a1ad348dc2a@infradead.org>
Date:   Thu, 8 Oct 2020 18:26:35 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     yulei.kernel@...il.com, 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: Re: [PATCH 35/35] Add documentation for dmemfs

On 10/8/20 12:54 AM, yulei.kernel@...il.com wrote:
> From: Yulei Zhang <yuleixzhang@...cent.com>
> 
> Introduce dmemfs.rst to document the basic usage of dmemfs.
> 

Please add dmemfs as an entry in Documentation/filesystems/index.rst also.

> 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,

                                                             usages,

> +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

                                     up. The details

> +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

Hm, is there a good reason for having both of these options?
Is one of them usable without the other one?
If not, there should only be one Kconfig option for DMEMFS.

> +
> +
> +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 ::

   Then

> +
> +    # 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
> 


-- 
~Randy