zram: add bd_stat statistics

bd_stat represents things that happened in the backing device.  Currently
it supports bd_counts, bd_reads and bd_writes which are helpful to
understand wearout of flash and memory saving.

[minchan@kernel.org: v4]
  Link: http://lkml.kernel.org/r/20181203024045.153534-7-minchan@kernel.org
Link: http://lkml.kernel.org/r/20181127055429.251614-7-minchan@kernel.orgSigned-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-block-zram

@@ -113,3 +113,11 @@ Contact:	Minchan Kim <minchan@kernel.org>
 Description:
 		The writeback file is write-only and trigger idle and/or
 		huge page writeback to backing device.
+
+What:		/sys/block/zram<id>/bd_stat
+Date:		November 2018
+Contact:	Minchan Kim <minchan@kernel.org>
+Description:
+		The bd_stat file is read-only and represents backing device's
+		statistics (bd_count, bd_reads, bd_writes) in a format
+		similar to block layer statistics file format.

Documentation/blockdev/zram.txt

@@ -221,6 +221,17 @@ line of text and contains the following stats separated by whitespace:
  pages_compacted  the number of pages freed during compaction
  huge_pages	  the number of incompressible pages
 
+File /sys/block/zram<id>/bd_stat
+
+The stat file represents device's backing device statistics. It consists of
+a single line of text and contains the following stats separated by whitespace:
+ bd_count	size of data written in backing device.
+		Unit: 4K bytes
+ bd_reads	the number of reads from backing device
+		Unit: 4K bytes
+ bd_writes	the number of writes to backing device
+		Unit: 4K bytes
+
 9) Deactivate:
 	swapoff /dev/zram0
 	umount /dev/zram1

drivers/block/zram/zram_drv.c

@@ -502,6 +502,7 @@ static unsigned long alloc_block_bdev(struct zram *zram)
 	if (test_and_set_bit(blk_idx, zram->bitmap))
 		goto retry;
 
+	atomic64_inc(&zram->stats.bd_count);
 	return blk_idx;
 }
 
@@ -511,6 +512,7 @@ static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
 
 	was_set = test_and_clear_bit(blk_idx, zram->bitmap);
 	WARN_ON_ONCE(!was_set);
+	atomic64_dec(&zram->stats.bd_count);
 }
 
 static void zram_page_end_io(struct bio *bio)
@@ -668,6 +670,7 @@ static ssize_t writeback_store(struct device *dev,
 			continue;
 		}
 
+		atomic64_inc(&zram->stats.bd_writes);
 		/*
 		 * We released zram_slot_lock so need to check if the slot was
 		 * changed. If there is freeing for the slot, we can catch it
@@ -757,6 +760,7 @@ static int read_from_bdev_sync(struct zram *zram, struct bio_vec *bvec,
 static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
 			unsigned long entry, struct bio *parent, bool sync)
 {
+	atomic64_inc(&zram->stats.bd_reads);
 	if (sync)
 		return read_from_bdev_sync(zram, bvec, entry, parent);
 	else
@@ -1013,6 +1017,25 @@ static ssize_t mm_stat_show(struct device *dev,
 	return ret;
 }
 
+#ifdef CONFIG_ZRAM_WRITEBACK
+static ssize_t bd_stat_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct zram *zram = dev_to_zram(dev);
+	ssize_t ret;
+
+	down_read(&zram->init_lock);
+	ret = scnprintf(buf, PAGE_SIZE,
+		"%8llu %8llu %8llu\n",
+		(u64)atomic64_read(&zram->stats.bd_count) * (PAGE_SHIFT - 12),
+		(u64)atomic64_read(&zram->stats.bd_reads) * (PAGE_SHIFT - 12),
+		(u64)atomic64_read(&zram->stats.bd_writes) * (PAGE_SHIFT - 12));
+	up_read(&zram->init_lock);
+
+	return ret;
+}
+#endif
+
 static ssize_t debug_stat_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
@@ -1033,6 +1056,9 @@ static ssize_t debug_stat_show(struct device *dev,
 
 static DEVICE_ATTR_RO(io_stat);
 static DEVICE_ATTR_RO(mm_stat);
+#ifdef CONFIG_ZRAM_WRITEBACK
+static DEVICE_ATTR_RO(bd_stat);
+#endif
 static DEVICE_ATTR_RO(debug_stat);
 
 static void zram_meta_free(struct zram *zram, u64 disksize)
@@ -1759,6 +1785,9 @@ static struct attribute *zram_disk_attrs[] = {
 #endif
 	&dev_attr_io_stat.attr,
 	&dev_attr_mm_stat.attr,
+#ifdef CONFIG_ZRAM_WRITEBACK
+	&dev_attr_bd_stat.attr,
+#endif
 	&dev_attr_debug_stat.attr,
 	NULL,
 };

drivers/block/zram/zram_drv.h

@@ -82,6 +82,11 @@ struct zram_stats {
 	atomic_long_t max_used_pages;	/* no. of maximum pages stored */
 	atomic64_t writestall;		/* no. of write slow paths */
 	atomic64_t miss_free;		/* no. of missed free */
+#ifdef	CONFIG_ZRAM_WRITEBACK
+	atomic64_t bd_count;		/* no. of pages in backing device */
+	atomic64_t bd_reads;		/* no. of reads from backing device */
+	atomic64_t bd_writes;		/* no. of writes from backing device */
+#endif
 };
 
 struct zram {