[PATCHv4 5/9] documentation: Add recompression documentation

[Sergey Senozhatsky]

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

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 Documentation/admin-guide/blockdev/zram.rst | 55 +++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index c73b16930449..c916c2b9da55 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 (in bytes) 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
 ===============
 
-- 
2.38.0.413.g74048e4d9e-goog