On Mon, Sep 22, 2014 at 02:17:33PM -0700, Andrew Morton wrote:
> On Mon, 22 Sep 2014 09:03:11 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> > Some zram usecase could want lower fullness than default 80 to
> > avoid unnecessary swapout-and-fail-recover overhead.
> >
> > A typical example is that mutliple swap with high piroirty
> > zram-swap and low priority HDD-swap so it could still enough
> > free swap space although one of swap devices is full(ie, zram).
> > It would be better to fail over to HDD-swap rather than failing
> > swap write to zram in this case.
> >
> > This patch exports fullness to user so user can control it
> > via the knob.
>
> Adding new userspace interfaces requires a pretty strong justification
> and it's unclear to me that this is being met. In fact the whole
> patchset reads like "we have some problem, don't know how to fix it so
> let's add a userspace knob to make it someone else's problem".
I explained rationale in 4/5's reply but if it's not enough or wrong,
please tell me.
>
> > index b13dc993291f..817738d14061 100644
> > --- a/Documentation/ABI/testing/sysfs-block-zram
> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > @@ -138,3 +138,13 @@ Description:
> > amount of memory ZRAM can use to store the compressed data. The
> > limit could be changed in run time and "0" means disable the
> > limit. No limit is the initial state. Unit: bytes
> > +
> > +What: /sys/block/zram<id>/fullness
> > +Date: August 2014
> > +Contact: Minchan Kim <minchan@xxxxxxxxxx>
> > +Description:
> > + The fullness file is read/write and specifies how easily
> > + zram become full state so if you set it to lower value,
> > + zram can reach full state easily compared to higher value.
> > + Curretnly, initial value is 80% but it could be changed.
> > + Unit: Percentage
>
> And I don't think that there is sufficient information here for a user
> to be able to work out what to do with this tunable.
I will put more words.
>
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -136,6 +136,37 @@ static ssize_t max_comp_streams_show(struct device *dev,
> > return scnprintf(buf, PAGE_SIZE, "%d\n", val);
> > }
> >
> > +static ssize_t fullness_show(struct device *dev,
> > + struct device_attribute *attr, char *buf)
> > +{
> > + int val;
> > + struct zram *zram = dev_to_zram(dev);
> > +
> > + down_read(&zram->init_lock);
> > + val = zram->fullness;
> > + up_read(&zram->init_lock);
>
> Did we really need to take a lock to display a value which became
> out-of-date as soon as we released that lock?
>
> > + return scnprintf(buf, PAGE_SIZE, "%d\n", val);
> > +}
> > +
> > +static ssize_t fullness_store(struct device *dev,
> > + struct device_attribute *attr, const char *buf, size_t len)
> > +{
> > + int err;
> > + unsigned long val;
> > + struct zram *zram = dev_to_zram(dev);
> > +
> > + err = kstrtoul(buf, 10, &val);
> > + if (err || val > 100)
> > + return -EINVAL;
>
> This overwrites the kstrtoul() return value.
Will fix.
Thanks for the reivew, Andrew.
--
Kind regards,
Minchan Kim
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