Date:	Wed, 30 Jul 2014 22:58:31 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Timofey Titovets <nefelim4ag@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Nitin Gupta <ngupta@...are.org>
Subject: Re: [PATCH v3 RESEND] zram: auto add new devices on demand

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@...il.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.

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@...are.org>");
> > --
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> 
> -- 
> Kind regards,
> Minchan Kim
> 
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