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Date:   Tue, 21 Jan 2020 06:58:58 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Zhiqiang Liu <liuzhiqiang26@huawei.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>
Subject: Re: [PATCH V3] brd: check and limit max_part par
Message-ID: <20200120225858.GB19571@ming.t460p>
References: <c8236e55-f64f-ef40-b394-8b7e86ce50df@huawei.com>
 <20200115022725.GA14585@ming.t460p>
 <ce5823ea-2183-90df-05b0-c02d1f654be3@huawei.com>
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On Mon, Jan 20, 2020 at 09:14:50PM +0800, Zhiqiang Liu wrote:
>=20
>=20
> On 2020/1/15 10:27, Ming Lei wrote:
>=20
> >=20
> >>  MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");
> >>
> >>  unsigned long rd_size =3D 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 =3D 1;
> >> -module_param(max_part, int, 0444);
> >> +static unsigned int max_part =3D 1;
> >> +module_param(max_part, uint, 0444);
> >=20
> > The above change isn't needed.
> Thanks for your suggestion.
> I will remove that in v4 patch.
> >=20
> >>  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		=3D RAMDISK_MAJOR;
> >> -	disk->first_minor	=3D 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 dev=
t.
> >> +	 */
> >> +	if ((i * disk->minors) & ~MINORMASK)
> >> +		disk->minors =3D 0;
> >> +	else
> >> +		disk->first_minor =3D i * disk->minors;
> >=20
> > The above looks a bit ugly, one nice way could be to change in
> > brd_alloc():
> >=20
> > 	disk =3D brd->brd_disk =3D alloc_disk(((i * max_part) & ~MINORMASK) =
?
> > 		0 : max_part);
>=20
> I will change it as your suggestion.
>=20
> >=20
> >>  	disk->fops		=3D &brd_fops;
> >>  	disk->private_data	=3D brd;
> >>  	disk->queue		=3D 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 =3D 1;
> >=20
> > zero rd_nr should work as expected, given user can create dev file vi=
a
> > mknod, and brd_probe() will be called for populate brd disk/queue whe=
n
> > the disk file is opened.
> >=20
> >> +static inline void brd_check_and_reset_par(void)
> >> +{
> >> + =A0 =A0 =A0 if (unlikely(!rd_nr))
> >> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 rd_nr =3D 1;
> >> +
> >> + =A0 =A0 =A0 if (unlikely(!max_part))
> >> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 max_part =3D 1;
> >=20
> > Another limit is that 'max_part' needs to be divided exactly by (1U <=
<
> > MINORBITS), something like:
> >=20
> > 	max_part =3D 1UL << fls(max_part)
>=20
> 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 MINOR=
MASK,
> 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