linux-kernel - Re: [PATCH v6 02/16] chardev: introduce cdev_get_by

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Date:   Thu, 25 Jul 2019 12:14:33 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()



On 2019-07-25 12:08 p.m., Greg Kroah-Hartman wrote:
> On Thu, Jul 25, 2019 at 11:53:20AM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2019-07-25 11:40 a.m., Greg Kroah-Hartman wrote:
>>> On Thu, Jul 25, 2019 at 11:23:21AM -0600, Logan Gunthorpe wrote:
>>>> cdev_get_by_path() attempts to retrieve a struct cdev from
>>>> a path name. It is analagous to blkdev_get_by_path().
>>>>
>>>> This will be necessary to create a nvme_ctrl_get_by_path()to
>>>> support NVMe-OF passthru.
>>>
>>> Ick, why?  Why would a cdev have a "pathname"?
>>
>> So we can go from "/dev/nvme0" (which points to a char device) to its
>> struct cdev and eventually it's struct nvme_ctrl. Doing it this way also
>> allows supporting symlinks that might be created by udev rules.
> 
> Why do you have a "string" within the kernel and are not using the
> normal open() call from userspace on the character device node on the
> filesystem in your namespace/mount/whatever?

NVMe-OF is configured using configfs. The target is specified by the
user writing a path to a configfs attribute. This is the way it works
today but with blkdev_get_by_path()[1]. For the passthru code, we need
to get a nvme_ctrl instead of a block_device, but the principal is the same.

> Where is this random string coming from?  

configfs

> Why is this so special that no
> one else has ever needed it?

People have needed the same functionality for block devices and
blkdev_get_by_path() has multiple users (iscsi, drbd, nvme-of, etc)
which are doing similar things. Nobody has needed to do the same with a
chardev until we wanted the NVMe-of to support targeting an NVMe
controller which is represented in userspace by a char device.

Logan


[1]
https://elixir.bootlin.com/linux/latest/source/drivers/nvme/target/io-cmd-bdev.c#L15