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

[Minchan Kim <minchan@kernel.org>]

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@google.com>
> ---
>  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,

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.
It adds locking dependency as well as performance overhead(I don't
think it's a good deal that scarfice hot path for rare event even
though it's not that big).