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From:   Brian Geffon <bgeffon@google.com>
Date:   Mon, 4 Oct 2021 14:40:52 -0400
Message-ID: <CADyq12xk-2Fhnf_rJQ70oC1_98OEBJqwxOt6z=PpJa5V=X3dFQ@mail.gmail.com>
Subject: Re: [PATCH] zram: Allow backing device to be assigned after init
To:     Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        linux-block@vger.kernel.org,
        Suleiman Souhlal <suleiman@google.com>,
        Jesse Barnes <jsbarnes@google.com>
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On Mon, Oct 4, 2021 at 2:29 PM Minchan Kim <minchan@kernel.org> 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@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,
>

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