Re: [PATCH v2 3/3] nvme: fire discovery log page change events to userspace

From: Sagi Grimberg <sagi@grimberg.me>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Keith Busch <keith.busch@intel.com>,
	linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	linux-nvme@lists.infradead.org,
	James Smart <james.smart@broadcom.com>
Subject: Re: [PATCH v2 3/3] nvme: fire discovery log page change events to userspace
Date: Tue, 3 Sep 2019 18:35:30 -0700	[thread overview]
Message-ID: <f9204955-a1b3-796a-dc4f-fd7af7946635@grimberg.me> (raw)
In-Reply-To: <20190902193341.GA28723@kroah.com>

>> Still don't understand how this is ok...
>>
>> I have /dev/nvme0 represents a network endpoint that I would discover
>> from, it is raising me an event to do a discovery operation (namely to
>> issue an ioctl to it) so my udev code calls a systemd script.
>>
>> By the time I actually get to do that, /dev/nvme0 represents now a new
>> network endpoint (where the event is no longer relevant to). I would
>> rather the discovery to explicitly fail than to give me something
>> different, so we pass some arguments that we verify in the operation.
>>
>> Its a stretch case, but it was raised by people as a potential issue.
> 
> Ok, and how do you handle this same thing for something like /dev/sda ?
> (hint, it isn't new, and is something we solved over a decade ago)
> 
> If you worry about stuff like this, use a persistant device naming
> scheme and have your device node be pointed to by a symlink.  Create
> that symlink by using the information in the initial 'ADD' uevent.
> 
> That way, when userspace opens the device node, it "knows" exactly which
> one it opens.  It sounds like you have a bunch of metadata to describe
> these uniquely, so pass that in the ADD event, not in some other crazy
> non-standard manner.

We could send these variables when adding the device and then validating
them using a rich-text-explanatory symlink. Seems slightly backwards to
me, but that would work too.

We create the char device using device_add (in nvme_init_subsystem),
I didn't find any way to append env variables to that ADD uevent.

Did you mean that we should add another flavor of device_add that
accepts char *envp_ext[]?

What exactly is the "standard manner" to pass these variables to
the chardev KOBJ_ADD uevent? All other examples I could find use
KOBJ_CHANGE to pass private stuff..

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