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

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Date:   Thu, 25 Jul 2019 20:10:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Logan Gunthorpe <logang@...tatee.com>
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 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.
> 
> This is very similar to blkdev_get_by_path() that lets regular NVMe-OF
> obtain the struct block_device from a path.
> 
> I didn't think this would be all that controversial.
> 
> > What is "NVMe-OF passthru"?  Why does a char device node have anything
> > to do with NVMe?
> 
> NVME-OF passthru is support for NVME over fabrics to directly target a
> regular NVMe controller and thus export an entire NVMe device to a
> remote system. We need to be able to tell the kernel which controller to
> use and IMO a path to the device file is the best way as it allows us to
> support symlinks created by udev.

open() in userspace handles symlinks just fine, what crazy interface
passes a string to try to find a char device node that is not open()?

And why do you need a char device at all anyway?  Is this just the
"normal" nvme controller's character device node?

> > We have way too many ways to abuse cdevs today, my long-term-wish has
> > always been to clean this interface up to make it more sane and unified,
> > and get rid of the "outliers" (all created at the time for a good
> > reason, that's not the problem.)  But to add "just one more" seems
> > really odd to me.
> 
> Well it doesn't seem all that much like an outlier to me.

Everyone is special, just like everyone else :)

Seriously, as no one else has ever needed this, you are an outlier.

thanks,

greg k-h