linux-kernel - Re: [PATCH v7 1/5] lightnvm: Support for Open-Channel SSDs

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Message-ID: <55E94301.3010501@cn.fujitsu.com>
Date:	Fri, 4 Sep 2015 15:06:41 +0800
From:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
To:	Matias Bjørling <m@...rling.me>,
	<hch@...radead.org>, <axboe@...com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-nvme@...ts.infradead.org>
CC:	<jg@...htnvm.io>, <Stephen.Bates@...s.com>,
	<keith.busch@...el.com>,
	Matias Bjørling <mb@...htnvm.io>
Subject: Re: [PATCH v7 1/5] lightnvm: Support for Open-Channel SSDs

On 09/02/2015 06:48 PM, Matias Bjørling wrote:
>>> +
>>> +    /* register with device with a supported BM */
>>> +    list_for_each_entry(bt, &nvm_bms, list) {
>>> +        ret = bt->register_bm(dev);
>>> +        if (ret < 0)
>>> +            goto err; /* initialization failed */
>>> +        if (ret > 0) {
>>> +            dev->bm = bt;
>>> +            break; /* successfully initialized */
>>> +        }
>>> +    }
>>
>> Why just search it from head to tail? Can user specific it
>> in nvm_create_target()?
>
> Hi Yang,
>
> Currently only the rrpc and a couple of out of tree block managers are
> built. The register_bm only tries to find a block manager that supports
> the device, when it finds it, that  one is initialized. It is an open
> question on how we choose the right block manager, e.g. a proprietary
> and a open-source block manager is in place. Priorities might be a way
> to go? or mark certain block managers as a catch all?
>
> Hopefully we will get away with only a single or two block managers in
> the future, so we won't have one for each type of device.
>
>>> +
>>> +    if (!ret) {
>>> +        pr_info("nvm: no compatible bm was found.\n");
>>> +        return 0;
>>> +    }
>>
>> If we allow nvm_device registered with no bm, we would get
>> a NULL pointer reference problem in later using.
>>
>
> Yes, definitely.

So here is a suggestion, register_bm again
if we found nvm_dev->bm == NULL in create_target(). And if it is still
NULL after that. return an error "nvm: no compatible bm was found"
and stop target creating. Otherwise, there would be a NULL Pointer
reference problem.

That's a real problem I met in my testing and I did this change
in my local using. I hope that's useful to you.

Thanx
Yang
> In the care that happens, I envision it should be
> possible to register a block manager after a device is loaded, and then
> any outstanding devices (which does not have a registered block
> manager), will be probed again.
>
>> As mentioned above, why we have to choose bm for nvm in nvm_register?
>
> Without a block manager, we don't know the structure of the device and
> how to interact with it. I want to initialize that as soon as possible.
> So that layers on top can start interacting.
>
>>
>> Thanx
>> Yang
> .
>

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