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Date:	Sun, 24 Aug 2014 23:40:50 -0400
From:	David Horner <ds2horner@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jerome Marchand <jmarchan@...hat.com>, juno.choi@....com,
	seungho1.park@....com, Luigi Semenzato <semenzato@...gle.com>,
	Nitin Gupta <ngupta@...are.org>,
	Seth Jennings <sjennings@...iantweb.net>,
	Dan Streetman <ddstreet@...e.org>
Subject: Re: [PATCH v4 3/4] zram: zram memory size limitation

On Sun, Aug 24, 2014 at 7:56 PM, Minchan Kim <minchan@...nel.org> 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@...nel.org> 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.
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.

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.
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@...nel.org>
>> > ---
>> >  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@...nel.org>
>> > +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@...ck.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
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>
> --
> Kind regards,
> Minchan Kim
--
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