Re: [PATCH v4 3/4] zram: zram memory size limitation

[Minchan Kim]

On Sun, Aug 24, 2014 at 11:40:50PM -0400, David Horner wrote:
> On Sun, Aug 24, 2014 at 7:56 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > Hello David,
> >
> > On Fri, Aug 22, 2014 at 06:55:38AM -0400, David Horner wrote:
> >> On Thu, Aug 21, 2014 at 8:42 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >> > Since zram has no control feature to limit memory usage,
> >> > it makes hard to manage system memrory.
> >> >
> >> > This patch adds new knob "mem_limit" via sysfs to set up the
> >> > a limit so that zram could fail allocation once it reaches
> >> > the limit.
> >> >
> >> > In addition, user could change the limit in runtime so that
> >> > he could manage the memory more dynamically.
> >> >
> >> - Default is no limit so it doesn't break old behavior.
> >> + Initial state is no limit so it doesn't break old behavior.
> >>
> >> I understand your previous post now.
> >>
> >> I was saying that setting to either a null value or garbage
> >>  (which is interpreted as zero by memparse(buf, NULL);)
> >> removes the limit.
> >>
> >> I think this is "surprise" behaviour and rather the null case should
> >> return  -EINVAL
> >> The test below should be "good enough" though not catching all garbage.
> >
> > Thanks for suggesting but as I said, it should be fixed in memparse itself,
> > not caller if it is really problem so I don't want to touch it in this
> > patchset. It's not critical for adding the feature.
> >
> 
> I've looked into the memparse function more since we talked.
> I do believe a wrapper function around it for the typical use by sysfs would
> be very valuable.

Agree.

> However, there is nothing wrong with memparse itself that needs to be fixed.
> 
> It does what it is documented to do very well (In My Uninformed Opinion).
> It provides everything that a caller needs to manage the token that it
> processes.
> It thus handles strings like "7,,5,8,,9" with the implied zeros.

Maybe strict_memparse would be better to protect such things so you
could find several places to clean it up.

> 
> The fact that other callers don't check the return pointer value to
> see if only a null
> string was processed, is not its fault.
> Nor that it may not be ideally suited to sysfs attributes; that other store
> functions use it in a given manner does not means that is correct -
> nor that it is
> incorrect for that "knob". Some attributes could be just as valid with
> null zeros.
> 
> And you are correct, to disambiguate the zero is not required for the
> limit feature.
> Your original patch which disallowed zero was full feature for mem_limit.
> It is the requested non-crucial feature to allow zero to reestablish
> the initial state
>  that benefits from distinguishing an explicit zero from a "default zero'
>  when garbage is written.
> 
> The final argument is that if we release this feature as is the undocumented
>  functionality could be relied upon, and when later fixed: user space breaks.

I don't get it. Why does it break userspace?
The sysfs-block-zram says "0" means disable the limit.
If someone writes *garabge* but work as if disabling the limit,
it's not a right thing and he already broke although it worked
so it would be not a problem if we fix later.
(ie, we don't need to take care of broken userspace)
Am I missing your point?

> They say getting API right is a difficult exercise. I suggest, if we
> don't insisting on
>  an explicit zero we have the API wrong.
> 
> I don't think you disagreed, just that the burden to get it correct
> lay elsewhere.
> 
> If that is the case it doesn't really matter, we cannot release this
> interface until
>  it is corrected wherever it must be.
> 
> And my zero check was a poor hack.
> 
> I should have explicitly checked the returned pointer value.
> 
> I will send that proposed revision, and hopefully you will consider it
> for inclusion.
> 
> 
> 
> 
> >>
> >> >
> >> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> >> > ---
> >> >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++
> >> >  Documentation/blockdev/zram.txt            | 24 ++++++++++++++---
> >> >  drivers/block/zram/zram_drv.c              | 41 ++++++++++++++++++++++++++++++
> >> >  drivers/block/zram/zram_drv.h              |  5 ++++
> >> >  4 files changed, 76 insertions(+), 4 deletions(-)
> >> >
> >> > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> >> > index 70ec992514d0..b8c779d64968 100644
> >> > --- a/Documentation/ABI/testing/sysfs-block-zram
> >> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> >> > @@ -119,3 +119,13 @@ Description:
> >> >                 efficiency can be calculated using compr_data_size and this
> >> >                 statistic.
> >> >                 Unit: bytes
> >> > +
> >> > +What:          /sys/block/zram<id>/mem_limit
> >> > +Date:          August 2014
> >> > +Contact:       Minchan Kim <minchan@xxxxxxxxxx>
> >> > +Description:
> >> > +               The mem_limit file is read/write and specifies the amount
> >> > +               of memory to be able to consume memory to store store
> >> > +               compressed data. The limit could be changed in run time
> >> > -               and "0" is default which means disable the limit.
> >> > +               and "0" means disable the limit. No limit is the initial state.
> >>
> >> there should be no default in the API.
> >
> > Thanks.
> >
> >>
> >> > +               Unit: bytes
> >> > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> >> > index 0595c3f56ccf..82c6a41116db 100644
> >> > --- a/Documentation/blockdev/zram.txt
> >> > +++ b/Documentation/blockdev/zram.txt
> >> > @@ -74,14 +74,30 @@ There is little point creating a zram of greater than twice the size of memory
> >> >  since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
> >> >  size of the disk when not in use so a huge zram is wasteful.
> >> >
> >> > -5) Activate:
> >> > +5) Set memory limit: Optional
> >> > +       Set memory limit by writing the value to sysfs node 'mem_limit'.
> >> > +       The value can be either in bytes or you can use mem suffixes.
> >> > +       In addition, you could change the value in runtime.
> >> > +       Examples:
> >> > +           # limit /dev/zram0 with 50MB memory
> >> > +           echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
> >> > +
> >> > +           # Using mem suffixes
> >> > +           echo 256K > /sys/block/zram0/mem_limit
> >> > +           echo 512M > /sys/block/zram0/mem_limit
> >> > +           echo 1G > /sys/block/zram0/mem_limit
> >> > +
> >> > +           # To disable memory limit
> >> > +           echo 0 > /sys/block/zram0/mem_limit
> >> > +
> >> > +6) Activate:
> >> >         mkswap /dev/zram0
> >> >         swapon /dev/zram0
> >> >
> >> >         mkfs.ext4 /dev/zram1
> >> >         mount /dev/zram1 /tmp
> >> >
> >> > -6) Stats:
> >> > +7) Stats:
> >> >         Per-device statistics are exported as various nodes under
> >> >         /sys/block/zram<id>/
> >> >                 disksize
> >> > @@ -96,11 +112,11 @@ size of the disk when not in use so a huge zram is wasteful.
> >> >                 compr_data_size
> >> >                 mem_used_total
> >> >
> >> > -7) Deactivate:
> >> > +8) Deactivate:
> >> >         swapoff /dev/zram0
> >> >         umount /dev/zram1
> >> >
> >> > -8) Reset:
> >> > +9) Reset:
> >> >         Write any positive value to 'reset' sysfs node
> >> >         echo 1 > /sys/block/zram0/reset
> >> >         echo 1 > /sys/block/zram1/reset
> >> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >> > index f0b8b30a7128..370c355eb127 100644
> >> > --- a/drivers/block/zram/zram_drv.c
> >> > +++ b/drivers/block/zram/zram_drv.c
> >> > @@ -122,6 +122,33 @@ static ssize_t max_comp_streams_show(struct device *dev,
> >> >         return scnprintf(buf, PAGE_SIZE, "%d\n", val);
> >> >  }
> >> >
> >> > +static ssize_t mem_limit_show(struct device *dev,
> >> > +               struct device_attribute *attr, char *buf)
> >> > +{
> >> > +       u64 val;
> >> > +       struct zram *zram = dev_to_zram(dev);
> >> > +
> >> > +       down_read(&zram->init_lock);
> >> > +       val = zram->limit_pages;
> >> > +       up_read(&zram->init_lock);
> >> > +
> >> > +       return scnprintf(buf, PAGE_SIZE, "%llu\n", val << PAGE_SHIFT);
> >> > +}
> >> > +
> >> > +static ssize_t mem_limit_store(struct device *dev,
> >> > +               struct device_attribute *attr, const char *buf, size_t len)
> >> > +{
> >> > +       u64 limit;
> >> > +       struct zram *zram = dev_to_zram(dev);
> >> > +
> >> > +       limit = memparse(buf, NULL);
> >>
> >>             if (limit = 0 && buf != "0")
> >>                   return  -EINVAL
> >>
> >> > +       down_write(&zram->init_lock);
> >> > +       zram->limit_pages = PAGE_ALIGN(limit) >> PAGE_SHIFT;
> >> > +       up_write(&zram->init_lock);
> >> > +
> >> > +       return len;
> >> > +}
> >> > +
> >> >  static ssize_t max_comp_streams_store(struct device *dev,
> >> >                 struct device_attribute *attr, const char *buf, size_t len)
> >> >  {
> >> > @@ -513,6 +540,14 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> >> >                 ret = -ENOMEM;
> >> >                 goto out;
> >> >         }
> >> > +
> >> > +       if (zram->limit_pages &&
> >> > +               zs_get_total_pages(meta->mem_pool) > zram->limit_pages) {
> >> > +               zs_free(meta->mem_pool, handle);
> >> > +               ret = -ENOMEM;
> >> > +               goto out;
> >> > +       }
> >> > +
> >> >         cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
> >> >
> >> >         if ((clen == PAGE_SIZE) && !is_partial_io(bvec)) {
> >> > @@ -617,6 +652,9 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >> >         struct zram_meta *meta;
> >> >
> >> >         down_write(&zram->init_lock);
> >> > +
> >> > +       zram->limit_pages = 0;
> >> > +
> >> >         if (!init_done(zram)) {
> >> >                 up_write(&zram->init_lock);
> >> >                 return;
> >> > @@ -857,6 +895,8 @@ static DEVICE_ATTR(initstate, S_IRUGO, initstate_show, NULL);
> >> >  static DEVICE_ATTR(reset, S_IWUSR, NULL, reset_store);
> >> >  static DEVICE_ATTR(orig_data_size, S_IRUGO, orig_data_size_show, NULL);
> >> >  static DEVICE_ATTR(mem_used_total, S_IRUGO, mem_used_total_show, NULL);
> >> > +static DEVICE_ATTR(mem_limit, S_IRUGO | S_IWUSR, mem_limit_show,
> >> > +               mem_limit_store);
> >> >  static DEVICE_ATTR(max_comp_streams, S_IRUGO | S_IWUSR,
> >> >                 max_comp_streams_show, max_comp_streams_store);
> >> >  static DEVICE_ATTR(comp_algorithm, S_IRUGO | S_IWUSR,
> >> > @@ -885,6 +925,7 @@ static struct attribute *zram_disk_attrs[] = {
> >> >         &dev_attr_orig_data_size.attr,
> >> >         &dev_attr_compr_data_size.attr,
> >> >         &dev_attr_mem_used_total.attr,
> >> > +       &dev_attr_mem_limit.attr,
> >> >         &dev_attr_max_comp_streams.attr,
> >> >         &dev_attr_comp_algorithm.attr,
> >> >         NULL,
> >> > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> >> > index e0f725c87cc6..b7aa9c21553f 100644
> >> > --- a/drivers/block/zram/zram_drv.h
> >> > +++ b/drivers/block/zram/zram_drv.h
> >> > @@ -112,6 +112,11 @@ struct zram {
> >> >         u64 disksize;   /* bytes */
> >> >         int max_comp_streams;
> >> >         struct zram_stats stats;
> >> > +       /*
> >> > +        * the number of pages zram can consume for storing compressed data
> >> > +        */
> >> > +       unsigned long limit_pages;
> >> > +
> >> >         char compressor[10];
> >> >  };
> >> >  #endif
> >> > --
> >> > 2.0.0
> >> >
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

-- 
Kind regards,
Minchan Kim
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