[PATCHv2 5/8] documentation: Add recompression documentation

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Document user-space visible device attributes that
are enabled by ZRAM_MULTI_COMP.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 Documentation/admin-guide/blockdev/zram.rst | 55 +++++++++++++++++++++
 drivers/block/zram/zram_drv.c               |  7 +--
 2 files changed, 57 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index c73b16930449..88957fcb6ad7 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -401,6 +401,61 @@ budget in next setting is user's job.
 If admin wants to measure writeback count in a certain period, they could
 know it via /sys/block/zram0/bd_stat's 3rd column.
 
+recompression
+-------------
+
+With CONFIG_ZRAM_MULTI_COMP, zram can recompress idle/huge pages using
+alternative (secondary) compression algorithm. The basic idea is that
+alternative compression algorithm can provide better compression ratio
+at a price of (potentially) slower compression/decompression speeds.
+Alternative compression algorithm can, for example, be more successful
+compressing huge pages (those that default algorithm failed to compress).
+Another application is idle pages recompression - pages that are cold and
+sit in the memory can be recompressed using more effective algorithm and,
+hence, reduce zsmalloc memory usage.
+
+With CONFIG_ZRAM_MULTI_COMP, zram will setup two compression algorithms
+per-CPU: primary and secondary ones. Primary zram compressor is explained
+in "3) Select compression algorithm", the secondary algorithm is configured
+in a similar way, using recomp_algorithm device attribute:
+
+Examples::
+
+	#show supported recompression algorithms
+	cat /sys/block/zramX/recomp_algorithm
+	zstd [lzo]
+
+	#select zstd recompression algorithm
+	echo zstd > /sys/block/zramX/recomp_algorithm
+
+Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
+which controls recompression:
+
+Examples::
+
+	#IDLE pages recompression is activated by `idle` mode
+	echo idle > /sys/block/zramX/recompress
+
+	#HUGE pages recompression is activated by `huge` mode
+	echo huge > /sys/block/zram0/recompress
+
+	#HUGE_IDLE pages recompression is activated by `huge_idle` mode
+	echo huge_idle > /sys/block/zramX/recompress
+
+The number of idle pages can be significant, so user-space can pass a size
+watermark value to the recompress knob, to filter out idle pages for
+recompression: zram will recompress only idle pages of equal or greater
+size:::
+
+	#recompress idle pages larger than 3000 bytes
+	echo 3000 > /sys/block/zramX/recompress
+
+	#recompress idle pages larger than 2000 bytes
+	echo 2000 > /sys/block/zramX/recompress
+
+Recompression is mostly focused on idle pages (except for huge pages
+recompression), so it works better in conjunction with memory tracking.
+
 memory tracking
 ===============
 
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index aacabaefb0c7..077a0bc60b8d 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1622,9 +1622,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
  *
  * Corresponding ZRAM slot should be locked.
  */
-static int zram_recompress(struct zram *zram,
-			   u32 index,
-			   struct page *page,
+static int zram_recompress(struct zram *zram, u32 index, struct page *page,
 			   int size_watermark)
 {
 	unsigned long handle_prev;
@@ -1707,8 +1705,7 @@ static int zram_recompress(struct zram *zram,
 
 static ssize_t recompress_store(struct device *dev,
 				struct device_attribute *attr,
-				const char *buf,
-				size_t len)
+				const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
 	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
-- 
2.38.0.rc1.362.ged0d419d3c-goog