+ documentation-zram-add-documentation-for-algorithm-parameters.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: Documentation/zram: add documentation for algorithm parameters
has been added to the -mm mm-unstable branch.  Its filename is
     documentation-zram-add-documentation-for-algorithm-parameters.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-zram-add-documentation-for-algorithm-parameters.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Documentation/zram: add documentation for algorithm parameters
Date: Fri, 12 Jul 2024 14:18:34 +0900

Document brief description of compression algorithms' parameters:
compression level and pre-trained dictionary.

Link: https://lkml.kernel.org/r/20240712051850.484318-24-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/admin-guide/blockdev/zram.rst |   45 ++++++++++++++----
 1 file changed, 36 insertions(+), 9 deletions(-)

--- a/Documentation/admin-guide/blockdev/zram.rst~documentation-zram-add-documentation-for-algorithm-parameters
+++ a/Documentation/admin-guide/blockdev/zram.rst
@@ -102,15 +102,33 @@ Examples::
 	#select lzo compression algorithm
 	echo lzo > /sys/block/zram0/comp_algorithm
 
-For the time being, the `comp_algorithm` content does not necessarily
-show every compression algorithm supported by the kernel. We keep this
-list primarily to simplify device configuration and one can configure
-a new device with a compression algorithm that is not listed in
-`comp_algorithm`. The thing is that, internally, ZRAM uses Crypto API
-and, if some of the algorithms were built as modules, it's impossible
-to list all of them using, for instance, /proc/crypto or any other
-method. This, however, has an advantage of permitting the usage of
-custom crypto compression modules (implementing S/W or H/W compression).
+For the time being, the `comp_algorithm` content shows only compression
+algorithms that are supported by zram.
+
+It is also possible to pass algorithm specific configuration parameters::
+
+	#set compression level to 8
+	echo "zstd level=8" > /sys/block/zram0/comp_algorithm
+
+Note that `comp_algorithm` also supports `algo=name` format::
+
+	#set compression level to 8
+	echo "algo=zstd level=8" > /sys/block/zram0/comp_algorithm
+
+Certain compression algorithms support pre-trained dictionaries, which
+significantly change algorithms' characteristics. In order to configure
+compression algorithm to use external pre-trained dictionary, pass full
+path to the dictionary along with other parameters::
+
+	#pass path to pre-trained dictionary
+	echo "algo=zstd dict=/etc/dictioary" > /sys/block/zram0/comp_algorithm
+
+Parameters are algorithm specific: not all algorithms support pre-trained
+dictionaries, not all algorithms support 'level'. Furthermore, for certain
+algorithms 'level' controls the compression level (the higher the value the
+better the compression ratio, it even can take negatives values for some
+algorithms), for other algorithms 'level' is acceleration level (the higher
+the value the lower the compression ratio).
 
 4) Set Disksize
 ===============
@@ -442,6 +460,15 @@ configuration:::
 	#select deflate recompression algorithm, priority 2
 	echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm
 
+The `recomp_algorithm` also supports algorithm configuration parameters, e.g.
+compression level and pre-trained dircionary::
+
+	#pass compression level
+	echo "algo=zstd level=8" > /sys/block/zramX/recomp_algorithm
+
+	#pass path to pre-trained dictionary
+	echo "algo=zstd dict=/etc/dictioary" > /sys/block/zramX/recomp_algorithm
+
 Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
 which controls recompression.
 
_

Patches currently in -mm which might be from senozhatsky@chromium.org are

lib-zstd-export-api-needed-for-dictionary-support.patch
lib-lz4hc-export-lz4_resetstreamhc-symbol.patch
lib-zstd-fix-null-deref-in-zstd_createcdict_advanced2.patch
zram-introduce-custom-comp-backends-api.patch
zram-add-lzo-and-lzorle-compression-backends-support.patch
zram-add-lz4-compression-backend-support.patch
zram-add-lz4hc-compression-backend-support.patch
zram-add-zstd-compression-backend-support.patch
zram-pass-estimated-src-size-hint-to-zstd.patch
zram-add-zlib-compression-backend-support.patch
zram-add-842-compression-backend-support.patch
zram-check-that-backends-array-has-at-least-one-backend.patch
zram-introduce-zcomp_params-structure.patch
zram-recalculate-zstd-compression-params-once.patch
zram-extend-comp_algorithm-attr-write-handling.patch
zram-add-support-for-dict-comp-config.patch
zram-introduce-zcomp_req-structure.patch
zram-introduce-zcomp_ctx-structure.patch
zram-move-immutable-comp-params-away-from-per-cpu-context.patch
zram-add-dictionary-support-to-lz4.patch
zram-add-dictionary-support-to-lz4hc.patch
zram-add-dictionary-support-to-zstd-backend.patch
documentation-zram-add-documentation-for-algorithm-parameters.patch