zram: support priority parameter in recompression

recompress device attribute supports alg=NAME parameter so that we can
specify only one particular algorithm we want to perform recompression
with. However, with algo params we now can have several exactly same
secondary algorithms but each with its own params tuning (e.g. priority 1
configured to use more aggressive level, and priority 2 configured to use
a pre-trained dictionary). Support priority=NUM parameter so that we can
correctly determine which secondary algorithm we want to use.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Sergey Senozhatsky <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Nick Terrell <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/blockdev/zram.rst

···
 algorithm that successfully compresses a particular page. Sometimes, however,
 it is convenient (and sometimes even necessary) to limit recompression to
 only one particular algorithm so that it will not try any other algorithms.
-This can be achieved by providing a algo=NAME parameter:::
+This can be achieved by providing a `algo` or `priority` parameter:::
 
 	#use zstd algorithm only (if registered)
 	echo "type=huge algo=zstd" > /sys/block/zramX/recompress
+
+	#use zstd algorithm only (if zstd was registered under priority 1)
+	echo "type=huge priority=1" > /sys/block/zramX/recompress
 
 memory tracking
 ===============

drivers/block/zram/zram_drv.c

···
 			algo = val;
 			continue;
 		}
+
+		if (!strcmp(param, "priority")) {
+			ret = kstrtouint(val, 10, &prio);
+			if (ret)
+				return ret;
+
+			if (prio == ZRAM_PRIMARY_COMP)
+				prio = ZRAM_SECONDARY_COMP;
+
+			prio_max = min(prio + 1, ZRAM_MAX_COMPS);
+			continue;
+		}
 	}
 
 	if (threshold >= huge_class_size)