Re: [PATCH V3] brd: check and limit max_part par

Ming Lei
2020-01-20T22:59:16Z

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