Replace scnprintf() with sysfs_emit() or sysfs_emit_at() in sysfs
*_show() functions in zram_drv.c to follow the kernel's guidelines
from Documentation/filesystems/sysfs.rst.

This improves consistency, safety, and makes the code easier to
maintain and update in the future.

Signed-off-by: Rahul Kumar <rk0006818@gmail.com>
---
 drivers/block/zram/zram_drv.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index fda7d8624889..a1f2f45d4b99 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -373,7 +373,7 @@ static ssize_t initstate_show(struct device *dev,
 	val = init_done(zram);
 	up_read(&zram->init_lock);
 
-	return scnprintf(buf, PAGE_SIZE, "%u\n", val);
+	return sysfs_emit(buf, "%u\n", val);
 }
 
 static ssize_t disksize_show(struct device *dev,
@@ -381,7 +381,7 @@ static ssize_t disksize_show(struct device *dev,
 {
 	struct zram *zram = dev_to_zram(dev);
 
-	return scnprintf(buf, PAGE_SIZE, "%llu\n", zram->disksize);
+	return sysfs_emit(buf, "%llu\n", zram->disksize);
 }
 
 static ssize_t mem_limit_store(struct device *dev,
@@ -532,7 +532,7 @@ static ssize_t writeback_limit_enable_show(struct device *dev,
 	spin_unlock(&zram->wb_limit_lock);
 	up_read(&zram->init_lock);
 
-	return scnprintf(buf, PAGE_SIZE, "%d\n", val);
+	return sysfs_emit(buf, "%d\n", val);
 }
 
 static ssize_t writeback_limit_store(struct device *dev,
@@ -567,7 +567,7 @@ static ssize_t writeback_limit_show(struct device *dev,
 	spin_unlock(&zram->wb_limit_lock);
 	up_read(&zram->init_lock);
 
-	return scnprintf(buf, PAGE_SIZE, "%llu\n", val);
+	return sysfs_emit(buf, "%llu\n", val);
 }
 
 static void reset_bdev(struct zram *zram)
@@ -1292,7 +1292,7 @@ static ssize_t recomp_algorithm_show(struct device *dev,
 		if (!zram->comp_algs[prio])
 			continue;
 
-		sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2, "#%d: ", prio);
+		sz += sysfs_emit_at(buf, sz, "#%d: ", prio);
 		sz += __comp_algorithm_show(zram, prio, buf + sz);
 	}
 
@@ -1365,7 +1365,7 @@ static ssize_t io_stat_show(struct device *dev,
 	ssize_t ret;
 
 	down_read(&zram->init_lock);
-	ret = scnprintf(buf, PAGE_SIZE,
+	ret = sysfs_emit(buf,
 			"%8llu %8llu 0 %8llu\n",
 			(u64)atomic64_read(&zram->stats.failed_reads),
 			(u64)atomic64_read(&zram->stats.failed_writes),
@@ -1395,7 +1395,7 @@ static ssize_t mm_stat_show(struct device *dev,
 	orig_size = atomic64_read(&zram->stats.pages_stored);
 	max_used = atomic_long_read(&zram->stats.max_used_pages);
 
-	ret = scnprintf(buf, PAGE_SIZE,
+	ret = sysfs_emit(buf,
 			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu %8llu %8llu\n",
 			orig_size << PAGE_SHIFT,
 			(u64)atomic64_read(&zram->stats.compr_data_size),
@@ -1420,8 +1420,8 @@ static ssize_t bd_stat_show(struct device *dev,
 	ssize_t ret;
 
 	down_read(&zram->init_lock);
-	ret = scnprintf(buf, PAGE_SIZE,
-		"%8llu %8llu %8llu\n",
+	ret = sysfs_emit(buf,
+			"%8llu %8llu %8llu\n",
 			FOUR_K((u64)atomic64_read(&zram->stats.bd_count)),
 			FOUR_K((u64)atomic64_read(&zram->stats.bd_reads)),
 			FOUR_K((u64)atomic64_read(&zram->stats.bd_writes)));
@@ -1439,7 +1439,7 @@ static ssize_t debug_stat_show(struct device *dev,
 	ssize_t ret;
 
 	down_read(&zram->init_lock);
-	ret = scnprintf(buf, PAGE_SIZE,
+	ret = sysfs_emit(buf,
 			"version: %d\n0 %8llu\n",
 			version,
 			(u64)atomic64_read(&zram->stats.miss_free));
@@ -2682,7 +2682,7 @@ static ssize_t hot_add_show(const struct class *class,
 
 	if (ret < 0)
 		return ret;
-	return scnprintf(buf, PAGE_SIZE, "%d\n", ret);
+	return sysfs_emit(buf, "%d\n", ret);
 }
 /* This attribute must be set to 0400, so CLASS_ATTR_RO() can not be used */
 static struct class_attribute class_attr_hot_add =
-- 
2.43.0

On Fri, 27 Jun 2025 09:22:56 +0530, Rahul Kumar wrote:
> Replace scnprintf() with sysfs_emit() or sysfs_emit_at() in sysfs
> *_show() functions in zram_drv.c to follow the kernel's guidelines
> from Documentation/filesystems/sysfs.rst.
> 
> This improves consistency, safety, and makes the code easier to
> maintain and update in the future.
> 
> [...]

Applied, thanks!

[1/1] block: zram: replace scnprintf() with sysfs_emit() in *_show() functions
      commit: 264a3fdab2365395e43d3a0b40162a29e61ffa22
[1/1] zram: pass buffer offset to zcomp_available_show()
      commit: e74a1c6a8e8af2422fce125c29b14f1d3fab5b5c

Best regards,
-- 
Jens Axboe

On (25/06/27 09:22), Rahul Kumar wrote:
[..]
>  static void reset_bdev(struct zram *zram)
> @@ -1292,7 +1292,7 @@ static ssize_t recomp_algorithm_show(struct device *dev,
>  		if (!zram->comp_algs[prio])
>  			continue;
>  
> -		sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2, "#%d: ", prio);
> +		sz += sysfs_emit_at(buf, sz, "#%d: ", prio);
>  		sz += __comp_algorithm_show(zram, prio, buf + sz);

Actually, there is a tiny bug in zcomp.  It's unrelated to this
patch, but I'll send a fix as followup, to avoid any conflicts.

On (25/06/27 09:22), Rahul Kumar wrote:
> Replace scnprintf() with sysfs_emit() or sysfs_emit_at() in sysfs
> *_show() functions in zram_drv.c to follow the kernel's guidelines
> from Documentation/filesystems/sysfs.rst.
> 
> This improves consistency, safety, and makes the code easier to
> maintain and update in the future.
> 
> Signed-off-by: Rahul Kumar <rk0006818@gmail.com>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

In most cases zcomp_available_show() is the only emitting
function that is called from sysfs read() handler, so it
assumes that there is a whole PAGE_SIZE buffer to work with.
There is an exception, however: recomp_algorithm_show().

In recomp_algorithm_show() we prepend the buffer with
priority number before we pass it to zcomp_available_show(),
so it cannot assume PAGE_SIZE anymore and must take
recomp_algorithm_show() modifications into consideration.
Therefore we need to pass buffer offset to zcomp_available_show().

Also convert it to use sysfs_emit_at(), to stay aligned
with the rest of zram's sysfs read() handlers.

On practice we are never even close to using the whole PAGE_SIZE
buffer, so that's not a critical bug, but still.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zcomp.c    | 15 +++++++--------
 drivers/block/zram/zcomp.h    |  2 +-
 drivers/block/zram/zram_drv.c |  9 +++++----
 3 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index d26a58c67e95..b1bd1daa0060 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -8,6 +8,7 @@
 #include <linux/sched.h>
 #include <linux/cpuhotplug.h>
 #include <linux/vmalloc.h>
+#include <linux/sysfs.h>
 
 #include "zcomp.h"
 
@@ -89,23 +90,21 @@ bool zcomp_available_algorithm(const char *comp)
 }
 
 /* show available compressors */
-ssize_t zcomp_available_show(const char *comp, char *buf)
+ssize_t zcomp_available_show(const char *comp, char *buf, ssize_t at)
 {
-	ssize_t sz = 0;
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(backends) - 1; i++) {
 		if (!strcmp(comp, backends[i]->name)) {
-			sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2,
-					"[%s] ", backends[i]->name);
+			at += sysfs_emit_at(buf, at, "[%s] ",
+					    backends[i]->name);
 		} else {
-			sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2,
-					"%s ", backends[i]->name);
+			at += sysfs_emit_at(buf, at, "%s ", backends[i]->name);
 		}
 	}
 
-	sz += scnprintf(buf + sz, PAGE_SIZE - sz, "\n");
-	return sz;
+	at += sysfs_emit_at(buf, at, "\n");
+	return at;
 }
 
 struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index 4acffe671a5e..eacfd3f7d61d 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -79,7 +79,7 @@ struct zcomp {
 
 int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node);
 int zcomp_cpu_dead(unsigned int cpu, struct hlist_node *node);
-ssize_t zcomp_available_show(const char *comp, char *buf);
+ssize_t zcomp_available_show(const char *comp, char *buf, ssize_t at);
 bool zcomp_available_algorithm(const char *comp);
 
 struct zcomp *zcomp_create(const char *alg, struct zcomp_params *params);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 385aaaa2531b..8acad3cc6e6e 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1225,12 +1225,13 @@ static void comp_algorithm_set(struct zram *zram, u32 prio, const char *alg)
 	zram->comp_algs[prio] = alg;
 }
 
-static ssize_t __comp_algorithm_show(struct zram *zram, u32 prio, char *buf)
+static ssize_t __comp_algorithm_show(struct zram *zram, u32 prio,
+				     char *buf, ssize_t at)
 {
 	ssize_t sz;
 
 	down_read(&zram->init_lock);
-	sz = zcomp_available_show(zram->comp_algs[prio], buf);
+	sz = zcomp_available_show(zram->comp_algs[prio], buf, at);
 	up_read(&zram->init_lock);
 
 	return sz;
@@ -1387,7 +1388,7 @@ static ssize_t comp_algorithm_show(struct device *dev,
 {
 	struct zram *zram = dev_to_zram(dev);
 
-	return __comp_algorithm_show(zram, ZRAM_PRIMARY_COMP, buf);
+	return __comp_algorithm_show(zram, ZRAM_PRIMARY_COMP, buf, 0);
 }
 
 static ssize_t comp_algorithm_store(struct device *dev,
@@ -1416,7 +1417,7 @@ static ssize_t recomp_algorithm_show(struct device *dev,
 			continue;
 
 		sz += sysfs_emit_at(buf, sz, "#%d: ", prio);
-		sz += __comp_algorithm_show(zram, prio, buf + sz);
+		sz += __comp_algorithm_show(zram, prio, buf, sz);
 	}
 
 	return sz;
-- 
2.50.0.727.gbf7dc18ff4-goog

On (25/06/27 16:18), Sergey Senozhatsky wrote:
> In most cases zcomp_available_show() is the only emitting
> function that is called from sysfs read() handler, so it
> assumes that there is a whole PAGE_SIZE buffer to work with.
> There is an exception, however: recomp_algorithm_show().
> 
> In recomp_algorithm_show() we prepend the buffer with
> priority number before we pass it to zcomp_available_show(),
> so it cannot assume PAGE_SIZE anymore and must take
> recomp_algorithm_show() modifications into consideration.
> Therefore we need to pass buffer offset to zcomp_available_show().
> 
> Also convert it to use sysfs_emit_at(), to stay aligned
> with the rest of zram's sysfs read() handlers.
> 
> On practice we are never even close to using the whole PAGE_SIZE
> buffer, so that's not a critical bug, but still.

Jens, do you feel like picking these 2 up or should I route
them to Andrew's tree?