Date:	Wed, 4 Mar 2015 09:18:06 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 5/8] zram: add dynamic device add/remove functionality

On (03/03/15 14:01), Andrew Morton wrote:
> > Introduce zram-control sysfs class, which has two sysfs attrs:
> > - zram_add      -- add a new specific (device_id) zram device
> > - zram_remove   -- remove a specific (device_id) zram device
> > 
> > Usage example:
> > 	# add a new specific zram device
> > 	echo 4 > /sys/class/zram-control/zram_add
> > 
> > 	# remove a specific zram device
> > 	echo 4 > /sys/class/zram-control/zram_remove
> > 
> > There is no automatic device_id generation, so user is expected to
> > provide one.
> > 
> > NOTE, there might be users who already depend on the fact that at
> > least zram0 device gets always created by zram_init(). Thus, due to
> > compatibility reasons, along with requested device_id (f.e. 5) zram0
> > will also be created.
> 
> This doesn't really explain why you think we need the patch.
> 
> - What's the current scheme for adding devices?
> 
> - What's wrong with it?
> 
> - Why is this better?

oh, yes. you are right.

We currently don't support on-demand device creation. The only way to have
N zram devices is to specify num_devices module parameter (default value 1).
That means that if, for some reason, at some point, user wants to have N + 1
devies he/she must umount all the existing devices, unload the module, load
the module passing num_devices equals to N + 1. And do this again, if needed.


	-ss


> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > ---
> >  Documentation/ABI/testing/sysfs-class-zram |  23 ++++++
> >  Documentation/blockdev/zram.txt            |  23 +++++-
> >  drivers/block/zram/zram_drv.c              | 120 +++++++++++++++++++++++++++++
> >  3 files changed, 162 insertions(+), 4 deletions(-)
> >  create mode 100644 Documentation/ABI/testing/sysfs-class-zram
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-class-zram b/Documentation/ABI/testing/sysfs-class-zram
> > new file mode 100644
> > index 0000000..99b2a1e
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-class-zram
> > @@ -0,0 +1,23 @@
> > +What:		/sys/class/zram-control/
> > +Date:		March 2015
> > +KernelVersion:	4.1
> > +Contact:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > +Description:
> > +		The zram-control/ class sub-directory belongs to zram
> > +		device class
> > +
> > +What:		/sys/class/zram-control/zram_add
> > +Date:		March 2015
> > +KernelVersion:	4.1
> > +Contact:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > +Description:
> > +		Add a specific /dev/zramX device, where X is a device_id
> > +		provided by user
> > +
> > +		What:		/sys/class/zram-control/zram_add
> > +Date:		March 2015
> > +KernelVersion:	4.1
> > +Contact:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > +Description:
> > +		Remove a specific /dev/zramX device, where X is a device_id
> > +		provided by user
> > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > index 7fcf9c6..4b140fa 100644
> > --- a/Documentation/blockdev/zram.txt
> > +++ b/Documentation/blockdev/zram.txt
> > @@ -19,7 +19,9 @@ Following shows a typical sequence of steps for using zram.
> >  1) Load Module:
> >  	modprobe zram num_devices=4
> >  	This creates 4 devices: /dev/zram{0,1,2,3}
> > -	(num_devices parameter is optional. Default: 1)
> > +
> > +num_devices parameter is optional and tells zram how many devices should be
> > +pre-created. Default: 1.
> >  
> >  2) Set max number of compression streams
> >  	Compression backend may use up to max_comp_streams compression streams,
> > @@ -97,7 +99,20 @@ size of the disk when not in use so a huge zram is wasteful.
> >  	mkfs.ext4 /dev/zram1
> >  	mount /dev/zram1 /tmp
> >  
> > -7) Stats:
> > +7) Add/remove zram devices
> > +
> > +zram provides a control interface, which enables dynamic (on-demand) device
> > +addition and removal.
> > +
> > +In order to add a new /dev/zramX device (where X is a unique device id)
> > +execute
> > +	echo X > /sys/class/zram-control/zram_add
> > +
> > +To remove the existing /dev/zramX device (where X is a device id)
> > +execute
> > +	echo X > /sys/class/zram-control/zram_remove
> > +
> > +8) Stats:
> >  	Per-device statistics are exported as various nodes under
> >  	/sys/block/zram<id>/
> >  		disksize
> > @@ -113,11 +128,11 @@ size of the disk when not in use so a huge zram is wasteful.
> >  		mem_used_total
> >  		mem_used_max
> >  
> > -8) Deactivate:
> > +9) Deactivate:
> >  	swapoff /dev/zram0
> >  	umount /dev/zram1
> >  
> > -9) Reset:
> > +10) 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 bab8b20..a457e16 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -33,16 +33,23 @@
> >  #include <linux/vmalloc.h>
> >  #include <linux/err.h>
> >  #include <linux/idr.h>
> > +#include <linux/sysfs.h>
> >  
> >  #include "zram_drv.h"
> >  
> >  static DEFINE_IDR(zram_index_idr);
> > +/* idr index must be protected */
> > +static DEFINE_MUTEX(zram_index_mutex);
> > +
> >  static int zram_major;
> >  static const char *default_compressor = "lzo";
> >  
> >  /* Module params (documentation at end) */
> >  static unsigned int num_devices = 1;
> >  
> > +#define ZRAM_CTL_ADD		1
> > +#define ZRAM_CTL_REMOVE		2
> > +
> >  #define ZRAM_ATTR_RO(name)						\
> >  static ssize_t name##_show(struct device *d,				\
> >  				struct device_attribute *attr, char *b)	\
> > @@ -1173,6 +1180,109 @@ static void zram_remove(struct zram *zram)
> >  	kfree(zram);
> >  }
> >  
> > +/* lookup if there is any device pointer that match the given device_id.
> > + * return device pointer if so, or ERR_PTR() otherwise. */
> > +static struct zram *zram_lookup(int dev_id)
> > +{
> > +	struct zram *zram;
> > +
> > +	zram = idr_find(&zram_index_idr, dev_id);
> > +	if (zram)
> > +		return zram;
> > +	return ERR_PTR(-ENODEV);
> > +}
> > +
> > +/* common zram-control add/remove handler */
> > +static int zram_control(int cmd, const char *buf)
> > +{
> > +	struct zram *zram;
> > +	int ret = -ENOSYS, err, dev_id;
> > +
> > +	/* dev_id is gendisk->first_minor, which is `int' */
> > +	ret = kstrtoint(buf, 10, &dev_id);
> > +	if (ret || dev_id < 0)
> > +		return ret;
> > +
> > +	mutex_lock(&zram_index_mutex);
> > +	zram = zram_lookup(dev_id);
> > +
> > +	switch (cmd) {
> > +	case ZRAM_CTL_ADD:
> > +		if (!IS_ERR(zram)) {
> > +			ret = -EEXIST;
> > +			break;
> > +		}
> > +		ret = zram_add(dev_id);
> > +		break;
> > +	case ZRAM_CTL_REMOVE:
> > +		if (IS_ERR(zram)) {
> > +			ret = PTR_ERR(zram);
> > +			break;
> > +		}
> > +
> > +		/* First, make ->disksize device attr RO, closing
> > +		 * ZRAM_CTL_REMOVE vs disksize_store() race window */
> > +		ret = sysfs_chmod_file(&disk_to_dev(zram->disk)->kobj,
> > +				&dev_attr_disksize.attr, S_IRUGO);
> > +		if (ret)
> > +			break;
> > +
> > +		ret = zram_reset_device(zram);
> > +		if (ret == 0) {
> > +			/* ->disksize is RO and there are no ->bd_openers */
> > +			zram_remove(zram);
> > +			break;
> > +		}
> > +
> > +		/* If there are still device bd_openers, try to make ->disksize
> > +		 * RW again and return. even if we fail to make ->disksize RW,
> > +		 * user still has RW ->reset attr. so it's possible to destroy
> > +		 * that device */
> > +		err = sysfs_chmod_file(&disk_to_dev(zram->disk)->kobj,
> > +				&dev_attr_disksize.attr,
> > +				S_IWUSR | S_IRUGO);
> > +		if (err)
> > +			ret = err;
> > +		break;
> > +	}
> > +	mutex_unlock(&zram_index_mutex);
> > +
> > +	return ret;
> > +}
> > +
> > +/* zram module control sysfs attributes */
> > +static ssize_t zram_add_store(struct class *class,
> > +			struct class_attribute *attr,
> > +			const char *buf,
> > +			size_t count)
> > +{
> > +	int ret = zram_control(ZRAM_CTL_ADD, buf);
> > +
> > +	return ret ? ret : count;
> > +}
> 
> The types here are confused.  `ret' is int, `count' is size_t, the
> function returns ssize_t and heaven knows what type "ret ?  ret :
> count" evaluates to!
> 
> 
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