Re: [PATCH v3 RESEND] zram: auto add new devices on demand

[Minchan Kim <minchan@kernel.org>]

On Wed, Jul 30, 2014 at 10:58:31PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (07/29/14 12:00), Minchan Kim wrote:
> > Hello Timofey,
> > 
> > Sorry for late response and thanks for suggesting new feature.
> > 
> > First of all, I'd like to know your usecase.
> > 
> > I don't mean I am against this feature and I tend to agree it would
> > be good if we can make new device dynamically but until now, I don't
> > hear any requirement like that. So we need compelling usecase to
> > justify maintainance overhead.
> > 
> > On Mon, Jul 21, 2014 at 01:46:14PM +0300, Timofey Titovets wrote:
> > > From: Timofey Titovets <nefelim4ag@gmail.com>
> > > 
> > > This add supporting of auto allocate new zram devices on demand,
> > > like loop devices
> > > 
> > > This working by following rules:
> > > 	- Pre create zram devices specified by num_device
> > > 	- if all device already in use -> add new free device
> > > 
> > > From v1 -> v2:
> > > Delete useless variable 'ret', added documentation for explain new
> > > zram behavior
> > > 
> > > From v2 -> v3
> > > Delete logic to destroy not used devices, for avoid concurrency issues
> > > Code refactoring for made patch small and clean as possible
> > > Patch can pass the test:
> > > 
> > > #!/bin/sh
> > > modprobe zram
> > > while true; do
> > >                 echo 10485760 > /sys/block/zram0/disksize&
> > >                 echo 1 > /sys/block/zram0/reset&
> > > done
> > > 
> > > Can be pulled from:
> > > https://github.com/Nefelim4ag/linux.git
> > > 
> > > Tested on 3.15.5-2-ARCH, can be applied on any kernel version
> > > after this patch 'zram: add LZ4 compression support' - https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=6e76668e415adf799839f0ab205142ad7002d260
> > > 
> > > ---
> > > 
> > > diff --git a/Documentation/blockdev/zram.txt
> > > b/Documentation/blockdev/zram.txt
> > > index 0595c3f..a090ac7 100644
> > > --- a/Documentation/blockdev/zram.txt
> > > +++ b/Documentation/blockdev/zram.txt
> > > @@ -21,6 +21,9 @@ Following shows a typical sequence of steps for
> > > using zram.
> > >  	This creates 4 devices: /dev/zram{0,1,2,3}
> > >  	(num_devices parameter is optional. Default: 1)
> > > 
> > > +	If all device in use kernel will create new zram device
> > > +	(between num_devices and 31)
> > > +
> > >  2) Set max number of compression streams
> > >  	Compression backend may use up to max_comp_streams compression streams,
> > >  	thus allowing up to max_comp_streams concurrent compression operations.
> > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > index 089e72c..cc78779 100644
> > > --- a/drivers/block/zram/zram_drv.c
> > > +++ b/drivers/block/zram/zram_drv.c
> > > @@ -43,6 +43,8 @@ static const char *default_compressor = "lzo";
> > >  /* Module params (documentation at end) */
> > >  static unsigned int num_devices = 1;
> > > 
> > > +static inline int zram_add_dev(void);
> > > +
> > >  #define ZRAM_ATTR_RO(name)						\
> > >  static ssize_t zram_attr_##name##_show(struct device *d,		\
> > >  				struct device_attribute *attr, char *b)	\
> > > @@ -168,6 +170,7 @@ static ssize_t comp_algorithm_store(struct device *dev,
> > >  		struct device_attribute *attr, const char *buf, size_t len)
> > >  {
> > >  	struct zram *zram = dev_to_zram(dev);
> > > +
> > 
> > unnecessary change
> > 
> > >  	down_write(&zram->init_lock);
> > >  	if (init_done(zram)) {
> > >  		up_write(&zram->init_lock);
> > > @@ -239,6 +242,7 @@ static struct zram_meta *zram_meta_alloc(u64 disksize)
> > >  {
> > >  	size_t num_pages;
> > >  	struct zram_meta *meta = kmalloc(sizeof(*meta), GFP_KERNEL);
> > > +
> > 
> > Ditto
> > 
> > >  	if (!meta)
> > >  		goto out;
> > > 
> > > @@ -374,6 +378,7 @@ static int zram_bvec_read(struct zram *zram,
> > > struct bio_vec *bvec,
> > >  	struct page *page;
> > >  	unsigned char *user_mem, *uncmem = NULL;
> > >  	struct zram_meta *meta = zram->meta;
> > > +
> > 
> > Ditto
> > 
> > >  	page = bvec->bv_page;
> > > 
> > >  	read_lock(&meta->tb_lock);
> > > @@ -607,6 +612,7 @@ static void zram_reset_device(struct zram *zram,
> > > bool reset_capacity)
> > >  	/* Free all pages that are still in this zram device */
> > >  	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> > >  		unsigned long handle = meta->table[index].handle;
> > > +
> > 
> > Ditto
> > 
> > >  		if (!handle)
> > >  			continue;
> > > 
> > > @@ -667,6 +673,7 @@ static ssize_t disksize_store(struct device *dev,
> > >  	zram->disksize = disksize;
> > >  	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> > >  	revalidate_disk(zram->disk);
> > > +	zram_add_dev();
> > 
> > Why do you add new device unconditionally?
> > Maybe we need new konb on sysfs or ioctl for adding new device?
> > Any thought, guys?
> 
> 
> speaking of the patch, frankly, I (almost) see no gain comparing to the
> existing functionality.
> 
> speaking of the idea. well, I'm not 100% convinced yet. the use cases I
> see around do not imply dynamic creation/resizing/etc. that said, I need to
> think about it.

It didn't persuade me, either.

Normally, distro have some config file for adding param at module loading
like /etc/modules. So, I think it should be done in there if someone want
to increase the number of zram devices.


> 
> if we end up adding this functionality I tend to vote for sysfs knob, just
> because it seems to be more user friendly than writing some magic INTs to
> ioctl-d fd.
> 
> 	-ss
> > 
> > >  	up_write(&zram->init_lock);
> > >  	return len;
> > > 
> > > @@ -954,6 +961,34 @@ static void destroy_device(struct zram *zram)
> > >  	blk_cleanup_queue(zram->queue);
> > >  }
> > > 
> > > +/*
> > > + * Automatically add empty zram device,
> > > + * if all created devices already initialized
> > > + */
> > > +static inline int zram_add_dev(void)
> > > +{
> > > +	int dev_id;
> > > +
> > > +	for (dev_id = 0; dev_id < num_devices; dev_id++) {
> > > +		if (!zram_devices[dev_id].disksize)
> > > +			return 0;
> > > +	}
> > > +
> > > +	if (max_num_devices <= num_devices) {
> > > +		pr_warn("Can't add zram%u, max device number reached\n", num_devices);
> > > +		return 1;
> > > +	}
> > > +
> > > +	if (create_device(&zram_devices[num_devices], num_devices)) {
> > > +		destroy_device(&zram_devices[num_devices]);
> > > +		return 1;
> > > +	}
> > > +	pr_info("Created zram%u device\n", num_devices);
> > > +	num_devices++;
> > 
> > Who protects num_devices race?
> > 
> > > +
> > > +	return 0;
> > > +}
> > 
> > There is only adding function. Where is removing function?
> > 
> > Sorry, I am on vacation tomorrow so pz, understand my late response.
> > 
> > > +
> > >  static int __init zram_init(void)
> > >  {
> > >  	int ret, dev_id;
> > > @@ -972,13 +1007,14 @@ static int __init zram_init(void)
> > >  		goto out;
> > >  	}
> > > 
> > > -	/* Allocate the device array and initialize each one */
> > > -	zram_devices = kzalloc(num_devices * sizeof(struct zram), GFP_KERNEL);
> > > +	/* Allocate the device array */
> > > +	zram_devices = kcalloc(max_num_devices, sizeof(struct zram), GFP_KERNEL);
> > >  	if (!zram_devices) {
> > >  		ret = -ENOMEM;
> > >  		goto unregister;
> > >  	}
> > > 
> > > +	/* Initialise zram{0..num_devices} */
> > >  	for (dev_id = 0; dev_id < num_devices; dev_id++) {
> > >  		ret = create_device(&zram_devices[dev_id], dev_id);
> > >  		if (ret)
> > > @@ -1025,7 +1061,7 @@ module_init(zram_init);
> > >  module_exit(zram_exit);
> > > 
> > >  module_param(num_devices, uint, 0);
> > > -MODULE_PARM_DESC(num_devices, "Number of zram devices");
> > > +MODULE_PARM_DESC(num_devices, "Number of pre created  zram devices");
> > > 
> > >  MODULE_LICENSE("Dual BSD/GPL");
> > >  MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > > the body of a message to majordomo@vger.kernel.org
> > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> > -- 
> > Kind regards,
> > Minchan Kim
> > 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

-- 
Kind regards,
Minchan Kim