[PATCH 5/7] zram: use crypto api to check alg availability

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

There is no way to get a string with all the crypto comp
algorithms supported by the crypto comp engine, so we need
to maintain our own backends list. At the same time we
additionally need to use crypto_has_comp() to make sure
that the user has requested a compression algorithm that is
recognized by the crypto comp engine. Relying on /proc/crypto
is not an options here, because it does not show not-yet-inserted
compression modules.

Example:

 modprobe zram
 cat /proc/crypto | grep -i lz4
 modprobe lz4
 cat /proc/crypto | grep -i lz4
name         : lz4
driver       : lz4-generic
module       : lz4

So the user can't tell exactly if the lz4 is really supported
from /proc/crypto output, unless someone or something has loaded
it.

This patch also adds crypto_has_comp() to zcomp_available_show().
We store all the compression algorithms names in zcomp's `backends'
array, regardless the CONFIG_CRYPTO_FOO configuration, but show
only those that are also supported by crypto engine. This helps
user to know the exact list of compression algorithms that can be
used.

Example:
  module lz4 is not loaded yet, but is supported by the crypto
  engine. /proc/crypto has no information on this module, while
  zram's `comp_algorithm' lists it:

 cat /proc/crypto | grep -i lz4

 cat /sys/block/zram0/comp_algorithm
[lzo] lz4 deflate lz4hc 842

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
 drivers/block/zram/zcomp.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 79b30d7..a8593e9 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -29,12 +29,16 @@ static const char * const backends[] = {
 static const char *find_backend(const char *compress)
 {
 	int i = 0;
+
 	while (backends[i]) {
 		if (sysfs_streq(compress, backends[i]))
 			break;
 		i++;
 	}
-	return backends[i];
+
+	if (backends[i] && crypto_has_comp(backends[i], 0, 0))
+		return backends[i];
+	return NULL;
 }
 
 static void zcomp_strm_free(struct zcomp *comp, struct zcomp_strm *zstrm)
@@ -74,14 +78,16 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
 	ssize_t sz = 0;
 	int i = 0;
 
-	while (backends[i]) {
+	for (; backends[i]; i++) {
+		if (!crypto_has_comp(backends[i], 0, 0))
+			continue;
+
 		if (!strcmp(comp, backends[i]))
 			sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2,
 					"[%s] ", backends[i]);
 		else
 			sz += scnprintf(buf + sz, PAGE_SIZE - sz - 2,
 					"%s ", backends[i]);
-		i++;
 	}
 	sz += scnprintf(buf + sz, PAGE_SIZE - sz, "\n");
 	return sz;
-- 
2.8.3.394.g3916adf