Re: [PATCH] zram: Allow backing device to be assigned after init

[Brian Geffon]

On Mon, Oct 4, 2021 at 2:29 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> On Fri, Oct 01, 2021 at 11:16:27AM -0700, Brian Geffon wrote:
> > There does not appear to be a technical reason to not
> > allow the zram backing device to be assigned after the
> > zram device is initialized.
> >
> > This change will allow for the backing device to be assigned
> > as long as no backing device is already assigned. In that
> > event backing_dev would return -EEXIST.
> >
> > Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> > ---
> >  drivers/block/zram/zram_drv.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index fcaf2750f68f..12b4555ee079 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -462,9 +462,9 @@ static ssize_t backing_dev_store(struct device *dev,
> >               return -ENOMEM;
> >
> >       down_write(&zram->init_lock);
> > -     if (init_done(zram)) {
> > -             pr_info("Can't setup backing device for initialized device\n");
> > -             err = -EBUSY;
> > +     if (zram->backing_dev) {
> > +             pr_info("Backing device is already assigned\n");
> > +             err = -EEXIST;
> >               goto out;
>
> Hi Brian,
>

Hi Minchan,

> I am worry about the inconsistency with other interface of current zram
> set up. They were supposed to set it up before zram disksize setting
> because it makes code more simple/maintainalbe in that we don't need
> to check some feature on the fly.
>
> Let's think about when zram extends the writeback of incompressible
> page on demand. The write path will need the backing_dev under
> down_read(&zarm->init_lock) or other conditional variable to check
> whether the feature is enabled or not on the fly.

I don't follow what you mean by that, writeback_store already holds
down_read(&zarm->init_lock).

Brian