Re: [PATCH 2/2] zram: user per-cpu compression streams

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (05/02/16 16:25), Sergey Senozhatsky wrote:
[..]
> > Trivial:
> > We could remove max_strm now and change description.
> 
> oh, yes.

how about something like this? remove max_comp_streams entirely, but
leave the attr. if we keep zram->max_comp_streams and return its value
(set by user space) from show() handler, we are techically lying;
because the actual number of streams is now num_online_cpus().


===8<===8<===

From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: [PATCH] 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.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 drivers/block/zram/zcomp.c    |  7 +------
 drivers/block/zram/zcomp.h    |  2 +-
 drivers/block/zram/zram_drv.c | 47 +++++++++++--------------------------------
 drivers/block/zram/zram_drv.h |  1 -
 4 files changed, 14 insertions(+), 43 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index d4159e4..d4de9cb 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -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);
@@ -211,7 +206,7 @@ void zcomp_destroy(struct zcomp *comp)
  * case of allocation error, or any other error potentially
  * returned by functions zcomp_strm_{multi,single}_create.
  */
-struct zcomp *zcomp_create(const char *compress, int max_strm)
+struct zcomp *zcomp_create(const char *compress)
 {
 	struct zcomp *comp;
 	struct zcomp_backend *backend;
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index aba8c21..ffd88cb 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -45,7 +45,7 @@ struct zcomp {
 ssize_t zcomp_available_show(const char *comp, char *buf);
 bool zcomp_available_algorithm(const char *comp);
 
-struct zcomp *zcomp_create(const char *comp, int max_strm);
+struct zcomp *zcomp_create(const char *comp);
 void zcomp_destroy(struct zcomp *comp);
 
 struct zcomp_strm *zcomp_strm_find(struct zcomp *comp);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index cad1751..817e511 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -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);
-
-	down_read(&zram->init_lock);
-	val = zram->max_comp_streams;
-	up_read(&zram->init_lock);
-
-	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;
-
-	down_write(&zram->init_lock);
-	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;
-out:
-	up_write(&zram->init_lock);
-	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);
@@ -1064,7 +1042,7 @@ static ssize_t disksize_store(struct device *dev,
 	if (!meta)
 		return -ENOMEM;
 
-	comp = zcomp_create(zram->compressor, zram->max_comp_streams);
+	comp = zcomp_create(zram->compressor);
 	if (IS_ERR(comp)) {
 		pr_err("Cannot initialise %s compressing backend\n",
 				zram->compressor);
@@ -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;
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 8e92339..06b1636 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -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 */
-- 
2.8.2