zram: check comp algorithm availability earlier

Improvement idea by Marcin Jabrzyk.

comp_algorithm_store() silently accepts any supplied algorithm name,
because zram performs algorithm availability check later, during the
device configuration phase in disksize_store() and emits the following
error:

  "zram: Cannot initialise %s compressing backend"

this error line is somewhat generic and, besides, can indicate a failed
attempt to allocate compression backend's working buffers.

add algorithm availability check to comp_algorithm_store():

  echo lzz > /sys/block/zram0/comp_algorithm
  -bash: echo: write error: Invalid argument

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Marcin Jabrzyk <m.jabrzyk@samsung.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zcomp.c

@@ -286,6 +286,11 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
 	return sz;
 }
 
+bool zcomp_available_algorithm(const char *comp)
+{
+	return find_backend(comp) != NULL;
+}
+
 bool zcomp_set_max_streams(struct zcomp *comp, int num_strm)
 {
 	return comp->set_max_streams(comp, num_strm);

drivers/block/zram/zcomp.h

@@ -51,6 +51,7 @@ 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);
 void zcomp_destroy(struct zcomp *comp);

drivers/block/zram/zram_drv.c

@@ -378,6 +378,9 @@ static ssize_t comp_algorithm_store(struct device *dev,
 	if (sz > 0 && zram->compressor[sz - 1] == '\n')
 		zram->compressor[sz - 1] = 0x00;
 
+	if (!zcomp_available_algorithm(zram->compressor))
+		len = -EINVAL;
+
 	up_write(&zram->init_lock);
 	return len;
 }