Re: [PATCH 5/8] zram: add dynamic device add/remove functionality

Andrew Morton
2015-03-03T22:01:49Z

On Tue, 3 Mar 2015 21:49:47 +0900 Sergey Senozhatsky <sergey.senozhatsky@gmail.com> 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?

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.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@gmail.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@gmail.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@gmail.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!