linux-kernel - Re: [PATCH v7 08/14] nvmet-core: allow one host per passthru-ctrl

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Message-ID: <5717f515-e051-c420-07b7-299bcfcd1f32@mellanox.com>
Date:   Sun, 18 Aug 2019 13:33:47 +0300
From:   Max Gurtovoy <maxg@...lanox.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        <linux-kernel@...r.kernel.org>, <linux-nvme@...ts.infradead.org>,
        <linux-block@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>
CC:     Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
        "Keith Busch" <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v7 08/14] nvmet-core: allow one host per passthru-ctrl


On 8/15/2019 7:06 PM, Logan Gunthorpe wrote:
>
> On 2019-08-15 6:36 a.m., Max Gurtovoy wrote:
>> On 8/2/2019 2:45 AM, Logan Gunthorpe wrote:
>>> This patch rejects any new connection to the passthru-ctrl if this
>>> controller is already connected to a different host. At the time of
>>> allocating the controller we check if the subsys associated with
>>> the passthru ctrl is already connected to a host and reject it
>>> if the hostnqn differs.
>> This is a big limitation.
>>
>> Are we plan to enable many front-end ctrl's to connect to the single
>> back-end ctrl in the future ?
> Honestly, I don't know that it's really necessary, but the limitation
> was requested by Sagi the first time this patch-set was submitted[1]
> citing unspecified user troubles. If there's consensus to remove the
> restriction I certainly can.
>
> Logan
>
> [1] http://lists.infradead.org/pipermail/linux-nvme/2018-April/016588.html

I don't understand why we don't limit a regular ctrl to single access 
and we do it for the PT ctrl.

I guess the block layer helps to sync between multiple access in 
parallel but we can do it as well.

Also, let's say you limit the access to this subsystem to 1 user, the 
bdev is still accessibly for local user and also you can create a 
different subsystem that will use this device (PT and non-PT ctrl).

Sagi,

can you explain the trouble you meant and how this limitation solve it ?