linux-kernel - Re: [PATCH V3] brd: check and limit max

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Message-ID: <20200120225858.GB19571@ming.t460p>
Date:   Tue, 21 Jan 2020 06:58:58 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Zhiqiang Liu <liuzhiqiang26@...wei.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mingfangsen <mingfangsen@...wei.com>, Guiyao <guiyao@...wei.com>,
        zhangsaisai <zhangsaisai@...wei.com>,
        "wubo (T)" <wubo40@...wei.com>
Subject: Re: [PATCH V3] brd: check and limit max_part par

On Mon, Jan 20, 2020 at 09:14:50PM +0800, Zhiqiang Liu wrote:
> 
> 
> 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.

Exact dividing is for reserving same minors for all disks with
RAMDISK_MAJOR, otherwise there is still chance to get same dev_t when
adding partitions.

Extended devt is for covering more disks, not related with 'max_part'.


Thanks,
Ming