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

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Date:   Fri, 26 Jul 2019 09:46:40 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Sagi Grimberg <sagi@...mberg.me>, Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Stephen Bates <sbates@...thlin.com>,
        linux-block@...r.kernel.org, Keith Busch <kbusch@...nel.org>,
        linux-fsdevel@...r.kernel.org, Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()



On 2019-07-25 10:29 p.m., Sagi Grimberg wrote:
> 
>>>>>>>> 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.
>>>>>>>
>>>>>>> Why isn't a fd being passed in there instead of a random string?
>>>>>>
>>>>>> I suppose we could echo a string of the file descriptor number there,
>>>>>> and look up the fd in the process' file descriptor table ...
>>>>>
>>>>> Assuming that there is a open handle somewhere out there...
>>>
>>> Yes, that would be a step backwards from an interface. The user would
>>> then need a special process to open the fd and pass it through configfs.
>>> They couldn't just do it with basic bash commands.
>>
>> First of all, they can, but... WTF not have filp_open() done right there?
>> Yes, by name.  With permission checks done.  And pick your object from
>> the
>> sodding struct file you'll get.
>>
>> What's the problem?  Why do you need cdev lookups, etc., when you are
>> dealing with files under your full control?  Just open them and use
>> ->private_data or whatever you set in ->open() to access the damn thing.
>> All there is to it...
> Oh this is so much simpler. There is really no point in using anything
> else. Just need to remember to compare f->f_op to what we expect to make
> sure that it is indeed the same device class.

Yes, that sounds like a good idea. I'll do this for v2.

Thanks,

Logan