Date:	Thu, 13 Feb 2014 13:53:29 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Jerome Marchand <jmarchan@...hat.com>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 1/4] zram: introduce compressing backend abstraction

Hello Sergey,

On Wed, Feb 12, 2014 at 10:39:33PM +0300, Sergey Senozhatsky wrote:
> ZRAM performs direct LZO compression algorithm calls, making it the one and
> only option. Introduce compressing backend abstraction zcomp in order to
> support multiple compression algorithms with the following set of operations:
>         .create
>         .destroy
>         .compress
>         .decompress
> 
> Schematically zram write() usually contains the following steps:
> 0) preparation (decompression of partioal IO, etc.)
> 1) lock buffer_lock mutex (protects meta compress buffers)
> 2) compress (using meta compress buffers)
> 3) alloc and map zs_pool object
> 4) copy compressed data (from meta compress buffers) to object allocated by 3)
> 5) free previous pool page, assign a new one
> 6) unlock buffer_lock mutex
> 
> As we can see, compressing buffers must remain untouched from 1) to 4),
> because, otherwise, concurrent write() can overwrite data. At the same
> time, zram_meta must be aware of a) specific compression algorithm
> memory requirements and b) necessary locking to protect compression
> buffers. Besides, zram holds buffer_lock almost through the whole write()
> function, making parallel compression impossible. To remove requirement
> a) new struct zcomp_strm introduced, which contain buffers required by
> compression algorithm. While struct zcomp implements zcomp_strm stream
> handling and locking by means of get() and put() semantics and removes
> requirement b) from zram meta. zcomp ->create() and ->destroy(), respectively,
> allocate and deallocate algorithm specific zcomp_strm `private' buffer.
> 
> Every zcomp has a list of idle zcomp_strm buffers, spinlock to protect idle
> list and wait queue, making it possible to perform parallel compressions.
> Each time zram issues a zcomp_strm_get() call, the following set of operations
> performed:
> - spin lock buffer_lock
> - if idle list is not empty, remove zcomp_strm from idle list, spin
>   unlock and return zcomp stream pointer to caller
> - if idle list is empty, current adds itself to wait queue. it will be
>   awaken by zcomp_strm_put() caller.
> 
> zcomp_strm_put():
> - spin lock buffer_lock
> - add zcomp stream to idle list
> - spin unlock, wake up sleeper
> 
> In other words, zcomp_strm_get() turns caller into exclusive user of a stream
> and zcomp_strm_put() makes a particular zcomp stream available.
> 
> Usage examples.
> 
> To initialize compressing backend:
> 	comp = zcomp_create(NAME) /* NAME e.g. "lzo" */
> 
> which initialises compressing backend if requested algorithm is supported.
> 
> Compress:
> 	zstrm = zcomp_strm_get(comp)
> 	zcomp_compress(comp, zstrm, src, src_len, &dst_len)
> 	[..] /* copy compressed data */
> 	zcomp_strm_put(comp, zstrm)
> 
> Decompress:
> 	zcomp_decompress(comp, src, src_len, dst, &dst_len);
> 
> Free compessing backend and its zcomp stream:
> 	zcomp_destroy(comp)
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  drivers/block/zram/zcomp.c     | 153 +++++++++++++++++++++++++++++++++++++++++
>  drivers/block/zram/zcomp.h     |  57 +++++++++++++++
>  drivers/block/zram/zcomp_lzo.c |  33 +++++++++
>  3 files changed, 243 insertions(+)
>  create mode 100644 drivers/block/zram/zcomp.c
>  create mode 100644 drivers/block/zram/zcomp.h
>  create mode 100644 drivers/block/zram/zcomp_lzo.c
> 
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> new file mode 100644
> index 0000000..3b4920f
> --- /dev/null
> +++ b/drivers/block/zram/zcomp.c
> @@ -0,0 +1,153 @@
> +/*
> + * Copyright (C) 2014 Sergey Senozhatsky.
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version
> + * 2 of the License, or (at your option) any later version.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/string.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +#include <linux/wait.h>
> +#include <linux/sched.h>
> +
> +#include "zcomp.h"
> +
> +extern struct zcomp_backend zcomp_lzo;
> +
> +static void zcomp_strm_free(struct zcomp *comp, struct zcomp_strm *zstrm)
> +{
> +	comp->backend->destroy(zstrm->private);
> +	free_pages((unsigned long)zstrm->buffer, 1);
> +	kfree(zstrm);
> +}
> +
> +/*
> + * allocate new zcomp_strm structure with ->private initialized by
> + * backend, return NULL on error
> + */
> +static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
> +{
> +	struct zcomp_strm *zstrm = kmalloc(sizeof(*zstrm), GFP_KERNEL);
> +	if (!zstrm)
> +		return NULL;
> +
> +	INIT_LIST_HEAD(&zstrm->list);
> +	/* algorithm specific working memory buffer */

Please don't use "buffer". As I said, it would have more other than buffer.

> +	zstrm->private = comp->backend->create();
> +	/*
> +	 * allocate 2 pages. 1 for compressed data, plus 1 extra for the
> +	 * case when compressed size is larger than the original one
> +	 */
> +	zstrm->buffer = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 1);
> +	if (!zstrm->private || !zstrm->buffer)
> +		goto fail;

I'm not fond of "goto" such simple error handling.

if (!zram->private || !zstrm->buffer) {
    zcmp_strm_free(comp, zstrm);
    zstrm = NULL;
}

return zstrm;

> +
> +	return zstrm;
> +fail:
> +	zcomp_strm_free(comp, zstrm);
> +	return NULL;
> +}
> +
> +/*
> + * get existing idle zcomp_strm or wait until other process release
> + * (zcomp_strm_put()) one for us
> + */
> +struct zcomp_strm *zcomp_strm_get(struct zcomp *comp)
> +{
> +	struct zcomp_strm *zstrm;
> +retry:
> +	spin_lock(&comp->buffer_lock);

Pz, rename buffer_lock with comp->strm_lock.

> +	if (list_empty(&comp->idle_strm)) {
> +		spin_unlock(&comp->buffer_lock);
> +		wait_event(comp->strm_wait,
> +				!list_empty(&comp->idle_strm));
> +		goto retry;
> +	}

Same reason: I don't like "goto"

while(1) {
    spin_lock
    if (list_empty()) {
        spin_unlock
        wait_event()
        continue;
    }
    
    zstrm = list_entry()
    list_del
    spin_unlock
    break;
}

return zstrm;

> +
> +	zstrm = list_entry(comp->idle_strm.next,
> +			struct zcomp_strm, list);
> +	list_del(&zstrm->list);
> +	spin_unlock(&comp->buffer_lock);
> +	return zstrm;
> +}
> +
> +/* add zcomp_strm back to idle list and wake up waiter (if any) */
> +void zcomp_strm_put(struct zcomp *comp, struct zcomp_strm *zstrm)
> +{
> +	spin_lock(&comp->buffer_lock);
> +	list_add_tail(&zstrm->list, &comp->idle_strm);

What I said "queue" in previous review is that I thougt stack might
be more cache-freindly than queue so list_add is proper than list_add_tail.
But it seems you have a reason to list_add_tail. If so, could you explain
what you expect?

> +	spin_unlock(&comp->buffer_lock);
> +
> +	wake_up(&comp->strm_wait);
> +}
> +
> +int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
> +		const unsigned char *src, size_t src_len, size_t *dst_len)
> +{
> +	return comp->backend->compress(src, src_len, zstrm->buffer,
> +			dst_len, zstrm->private);
> +}

We need src_len? it's always PAGE_SIZE.

> +
> +int zcomp_decompress(struct zcomp *comp, const unsigned char *src,
> +		size_t src_len, unsigned char *dst, size_t *dst_len)
> +{
> +	return comp->backend->decompress(src, src_len, dst, dst_len);

We need dst_len? It's always PAGE_SIZE, too.

> +}
> +
> +/* free allocated zcomp_strm buffers and zcomp */
> +void zcomp_destroy(struct zcomp *comp)
> +{
> +	struct zcomp_strm *zstrm;
> +	while (!list_empty(&comp->idle_strm)) {
> +		zstrm = list_entry(comp->idle_strm.next,
> +				struct zcomp_strm, list);
> +		list_del(&zstrm->list);
> +		zcomp_strm_free(comp, zstrm);
> +	}
> +	kfree(comp);
> +}
> +
> +static struct zcomp_backend *find_backend(const char *compress)
> +{
> +	if (sysfs_streq(compress, "lzo"))
> +		return &zcomp_lzo;
> +	return NULL;
> +}
> +
> +/*
> + * search available compressors for requested algorithm.
> + * allocate new zcomp and initialize it. return NULL
> + * if requested algorithm is not supported or in case
> + * of init error
> + */
> +struct zcomp *zcomp_create(const char *compress)
> +{
> +	struct zcomp *comp;
> +	struct zcomp_backend *backend;
> +	struct zcomp_strm *zstrm;
> +
> +	backend = find_backend(compress);
> +	if (!backend)
> +		return NULL;
> +
> +	comp = kmalloc(sizeof(struct zcomp), GFP_KERNEL);
> +	if (!comp)
> +		return NULL;
> +
> +	comp->backend = backend;
> +	spin_lock_init(&comp->buffer_lock);
> +	INIT_LIST_HEAD(&comp->idle_strm);
> +	init_waitqueue_head(&comp->strm_wait);
> +
> +	zstrm = zcomp_strm_alloc(comp);
> +	if (!zstrm) {
> +		zcomp_destroy(comp);
> +		return NULL;
> +	}
> +	list_add_tail(&zstrm->list, &comp->idle_strm);

It's first adding to empty list so there is no difference
between list_add and list_add_tail then, let's use list_add.

> +	return comp;
> +}
> diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
> new file mode 100644
> index 0000000..213e503
> --- /dev/null
> +++ b/drivers/block/zram/zcomp.h
> @@ -0,0 +1,57 @@
> +/*
> + * Copyright (C) 2014 Sergey Senozhatsky.
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version
> + * 2 of the License, or (at your option) any later version.
> + */
> +
> +#ifndef _ZCOMP_H_
> +#define _ZCOMP_H_
> +
> +#include <linux/types.h>
> +#include <linux/spinlock.h>
> +
> +struct zcomp_strm {
> +	void *buffer;     /* compression/decompression buffer */
> +	void *private;      /* algorithm workmem */

Remove workmem.

> +	struct list_head list;
> +};
> +
> +/* static compression backend */
> +struct zcomp_backend {
> +	int (*compress)(const unsigned char *src, size_t src_len,
> +			unsigned char *dst, size_t *dst_len, void *wrkmem);

Rename wrkmem.

> +
> +	int (*decompress)(const unsigned char *src, size_t src_len,
> +			unsigned char *dst, size_t *dst_len);
> +
> +	void * (*create)(void);
> +	void (*destroy)(void *private);
> +
> +	const char *name;
> +};
> +
> +/* dynamic per-device compression frontend */
> +struct zcomp {
> +	/* protect strm list */
> +	spinlock_t buffer_lock;

Pz, rename.

> +	/* list of available strms */
> +	struct list_head idle_strm;
> +	wait_queue_head_t strm_wait;
> +	struct zcomp_backend *backend;
> +};
> +
> +struct zcomp *zcomp_create(const char *comp);
> +void zcomp_destroy(struct zcomp *comp);
> +
> +struct zcomp_strm *zcomp_strm_get(struct zcomp *comp);
> +void zcomp_strm_put(struct zcomp *comp, struct zcomp_strm *zsrtm);
                                                              zstrm
> +
> +int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
> +		const unsigned char *src, size_t src_len, size_t *dst_len);
> +
> +int zcomp_decompress(struct zcomp *comp, const unsigned char *src,
> +		size_t src_len, unsigned char *dst, size_t *dst_len);
> +#endif /* _ZCOMP_H_ */
> diff --git a/drivers/block/zram/zcomp_lzo.c b/drivers/block/zram/zcomp_lzo.c
> new file mode 100644
> index 0000000..2b23771
> --- /dev/null
> +++ b/drivers/block/zram/zcomp_lzo.c
> @@ -0,0 +1,33 @@
> +/*
> + * Copyright (C) 2014 Sergey Senozhatsky.
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version
> + * 2 of the License, or (at your option) any later version.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/slab.h>
> +#include <linux/lzo.h>
> +
> +#include "zcomp.h"
> +
> +static void * lzo_create(void)
> +{
> +	return kzalloc(LZO1X_MEM_COMPRESS, GFP_KERNEL);
> +}
> +
> +static void lzo_destroy(void *private)
> +{
> +	kfree(private);
> +}
> +
> +extern struct zcomp_backend zcomp_lzo;
> +struct zcomp_backend zcomp_lzo = {
> +	.compress = lzo1x_1_compress,
> +	.decompress = lzo1x_decompress_safe,
> +	.create = lzo_create,
> +	.destroy = lzo_destroy,
> +	.name = "lzo",
> +};
> -- 
> 1.9.0.rc3.256.g4af3ebd
> 
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-- 
Kind regards,
Minchan Kim
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