Commit 43209ea2 authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Linus Torvalds

zram: remove max_comp_streams internals

Remove the internal part of max_comp_streams interface, since we
switched to per-cpu streams.  We will keep RW max_comp_streams attr
around, because:

a) we may (silently) switch back to idle compression streams list and
   don't want to disturb user space

b) max_comp_streams attr must wait for the next 'lay off cycle'; we
   give user space 2 years to adjust before we remove/downgrade the attr,
   and there are already several attrs scheduled for removal in 4.11, so
   it's too late for max_comp_streams.

This slightly change a user visible behaviour:

- First, reading from max_comp_stream file now will always return the
  number of online CPUs.

- Second, writing to max_comp_stream will not take any effect.

Link: default avatarSergey Senozhatsky <>
Cc: Minchan Kim <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent d34f6157
......@@ -59,27 +59,16 @@ 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,
thus allowing up to max_comp_streams concurrent compression operations.
By default, compression backend uses single compression stream.
#show max compression streams number
Regardless the value passed to this attribute, ZRAM will always
allocate multiple compression streams - one per online CPUs - thus
allowing several concurrent compression operations. The number of
allocated compression streams goes down when some of the CPUs
become offline. There is no single-compression-stream mode anymore,
unless you are running a UP system or has only 1 CPU online.
To find out how many streams are currently available:
cat /sys/block/zram0/max_comp_streams
#set max compression streams number to 3
echo 3 > /sys/block/zram0/max_comp_streams
In order to enable compression backend's multi stream support max_comp_streams
must be initially set to desired concurrency level before ZRAM device
initialisation. Once the device initialised as a single stream compression
backend (max_comp_streams equals to 1), you will see error if you try to change
the value of max_comp_streams because single stream compression backend
implemented as a special case by lock overhead issue and does not support
dynamic max_comp_streams. Only multi stream backend supports dynamic
max_comp_streams adjustment.
3) Select compression algorithm
Using comp_algorithm device attribute one can see available and
currently selected (shown in square brackets) compression algorithms,
......@@ -95,11 +95,6 @@ bool zcomp_available_algorithm(const char *comp)
return find_backend(comp) != NULL;
bool zcomp_set_max_streams(struct zcomp *comp, int num_strm)
return true;
struct zcomp_strm *zcomp_strm_find(struct zcomp *comp)
return *get_cpu_ptr(comp->stream);
......@@ -304,46 +304,25 @@ static ssize_t mem_used_max_store(struct device *dev,
return len;
* We switched to per-cpu streams and this attr is not needed anymore.
* However, we will keep it around for some time, because:
* a) we may revert per-cpu streams in the future
* b) it's visible to user space and we need to follow our 2 years
* retirement rule; but we already have a number of 'soon to be
* altered' attrs, so max_comp_streams need to wait for the next
* layoff cycle.
static ssize_t max_comp_streams_show(struct device *dev,
struct device_attribute *attr, char *buf)
int val;
struct zram *zram = dev_to_zram(dev);
val = zram->max_comp_streams;
return scnprintf(buf, PAGE_SIZE, "%d\n", val);
return scnprintf(buf, PAGE_SIZE, "%d\n", num_online_cpus());
static ssize_t max_comp_streams_store(struct device *dev,
struct device_attribute *attr, const char *buf, size_t len)
int num;
struct zram *zram = dev_to_zram(dev);
int ret;
ret = kstrtoint(buf, 0, &num);
if (ret < 0)
return ret;
if (num < 1)
return -EINVAL;
if (init_done(zram)) {
if (!zcomp_set_max_streams(zram->comp, num)) {
pr_info("Cannot change max compression streams\n");
ret = -EINVAL;
goto out;
zram->max_comp_streams = num;
ret = len;
return ret;
return len;
static ssize_t comp_algorithm_show(struct device *dev,
......@@ -1035,7 +1014,6 @@ static void zram_reset_device(struct zram *zram)
/* Reset stats */
memset(&zram->stats, 0, sizeof(zram->stats));
zram->disksize = 0;
zram->max_comp_streams = 1;
set_capacity(zram->disk, 0);
part_stat_set_all(&zram->disk->part0, 0);
......@@ -1299,7 +1277,6 @@ static int zram_add(void)
strlcpy(zram->compressor, default_compressor, sizeof(zram->compressor));
zram->meta = NULL;
zram->max_comp_streams = 1;
pr_info("Added device: %s\n", zram->disk->disk_name);
return device_id;
......@@ -102,7 +102,6 @@ struct zram {
* the number of pages zram can consume for storing compressed data
unsigned long limit_pages;
int max_comp_streams;
struct zram_stats stats;
atomic_t refcount; /* refcount for zram_meta */
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