linux-kernel - Re: [PATCH 1/8] dax: add region-available-size attribute

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Date:   Wed, 14 Dec 2016 22:47:00 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] dax: add region-available-size attribute

On Wed, Dec 14, 2016 at 7:53 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Wed, Dec 14, 2016 at 6:38 AM, Johannes Thumshirn <jthumshirn@...e.de> wrote:
>> Hi Dan,
>>
>> On Sat, Dec 10, 2016 at 10:28:30PM -0800, Dan Williams wrote:
>>> In preparation for a facility that enables dax regions to be
>>> sub-divided, introduce a 'dax/available_size' attribute.  This attribute
>>> appears under the parent device that registered the device-dax region,
>>> and it assumes that the device-dax-core owns the driver-data for that
>>> device.
>>>
>>> 'dax/available_size' adjusts dynamically as dax-device instances are
>>> registered and unregistered.
>>>
>>> As a side effect of using __request_region() to reserve capacity from
>>> the dax_region we now track pointers to those returned resources rather
>>> than duplicating the passed in resource array.
>>>
>>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>>> ---
>>
>> [...]
>>
>>> +static const struct attribute_group *dax_region_attribute_groups[] = {
>>> +     &dax_region_attribute_group,
>>> +     NULL,
>>>  };
>>>
>>>  static struct inode *dax_alloc_inode(struct super_block *sb)
>>> @@ -200,12 +251,27 @@ void dax_region_put(struct dax_region *dax_region)
>>>  }
>>>  EXPORT_SYMBOL_GPL(dax_region_put);
>>>
>>> +
>>
>> Stray extra newline?
>>
>> [...]
>>
>>>  struct dax_region *alloc_dax_region(struct device *parent, int region_id,
>>>               struct resource *res, unsigned int align, void *addr,
>>>               unsigned long pfn_flags)
>>>  {
>>>       struct dax_region *dax_region;
>>>
>>> +     if (dev_get_drvdata(parent)) {
>>> +             dev_WARN(parent, "dax core found drvdata already in use\n");
>>> +             return NULL;
>>> +     }
>>> +
>>
>> My first thought was, it might be interesting to see who already claimed
>> the drvdata. Then I figured, how are multiple sub-regions of a dax-device
>> supposed to work? What am I missing here?
>
> This is a check similar to the -EBUSY return you would get from
> request_mem_region(). In fact if all dax drivers are correctly calling
> request_mem_region() before alloc_dax_region() then it would be
> impossible for this check to ever fire. It's already impossible
> because there's only one dax driver upstream (dax_pmem). It's not
> really benefiting the kernel at all until we have multiple dax
> drivers, I'll remove it.

No, I went to go delete this and remembered the real reason this was
added. A device driver that calls alloc_dax_region() commits to
letting the dax core own dev->driver_data. Since this wasn't even
clear to me, I'll go fix up the comment.