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Subject: Re: [PATCH V3] brd: check and limit max_part par
To:     Ming Lei <ming.lei@redhat.com>
CC:     Jens Axboe <axboe@kernel.dk>, <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mingfangsen <mingfangsen@huawei.com>, Guiyao <guiyao@huawei.com>,
        zhangsaisai <zhangsaisai@huawei.com>,
        "wubo (T)" <wubo40@huawei.com>
References: <c8236e55-f64f-ef40-b394-8b7e86ce50df@huawei.com>
 <20200115022725.GA14585@ming.t460p>
From:   Zhiqiang Liu <liuzhiqiang26@huawei.com>
Message-ID: <ce5823ea-2183-90df-05b0-c02d1f654be3@huawei.com>
Date:   Mon, 20 Jan 2020 21:14:50 +0800
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On 2020/1/15 10:27, Ming Lei wrote:

> 
>>  MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");
>>
>>  unsigned long rd_size = CONFIG_BLK_DEV_RAM_SIZE;
>>  module_param(rd_size, ulong, 0444);
>>  MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");
>>
>> -static int max_part = 1;
>> -module_param(max_part, int, 0444);
>> +static unsigned int max_part = 1;
>> +module_param(max_part, uint, 0444);
> 
> The above change isn't needed.
Thanks for your suggestion.
I will remove that in v4 patch.
> 
>>  MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");
>>
>>  MODULE_LICENSE("GPL");
>> @@ -393,7 +393,14 @@ static struct brd_device *brd_alloc(int i)
>>  	if (!disk)
>>  		goto out_free_queue;
>>  	disk->major		= RAMDISK_MAJOR;
>> -	disk->first_minor	= i * max_part;
>> +	/*
>> +	 * Clear .minors when running out of consecutive minor space since
>> +	 * GENHD_FL_EXT_DEVT is set, and we can allocate from extended devt.
>> +	 */
>> +	if ((i * disk->minors) & ~MINORMASK)
>> +		disk->minors = 0;
>> +	else
>> +		disk->first_minor = i * disk->minors;
> 
> The above looks a bit ugly, one nice way could be to change in
> brd_alloc():
> 
> 	disk = brd->brd_disk = alloc_disk(((i * max_part) & ~MINORMASK) ?
> 		0 : max_part);

I will change it as your suggestion.

> 
>>  	disk->fops		= &brd_fops;
>>  	disk->private_data	= brd;
>>  	disk->queue		= brd->brd_queue;
>> @@ -468,6 +475,21 @@ static struct kobject *brd_probe(dev_t dev, int *part, void *data)
>>  	return kobj;
>>  }
>>
>> +static inline void brd_check_and_reset_par(void)
>> +{
>> +	if (unlikely(!rd_nr))
>> +		rd_nr = 1;
> 
> zero rd_nr should work as expected, given user can create dev file via
> mknod, and brd_probe() will be called for populate brd disk/queue when
> the disk file is opened.
> 
>> +static inline void brd_check_and_reset_par(void)
>> +{
>> +       if (unlikely(!rd_nr))
>> +               rd_nr = 1;
>> +
>> +       if (unlikely(!max_part))
>> +               max_part = 1;
> 
> Another limit is that 'max_part' needs to be divided exactly by (1U <<
> MINORBITS), something like:
> 
> 	max_part = 1UL << fls(max_part)

Do we have to limit that 'max_part' needs to be divided exactly by (1U <<
> MINORBITS)? As your suggestion, the i * max_part is larger than MINORMASK,
we can allocate from extended devt.

Thanks,
Zhiqiang Liu