linux-kernel - [PATCH] Documentation: zram: update IDLE pages tracking documentation

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Message-ID: <20250416042833.3858827-1-senozhatsky@chromium.org>
Date: Wed, 16 Apr 2025 13:27:59 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: Minchan Kim <minchan@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Shin Kawamura <kawasin@...gle.com>
Subject: [PATCH] Documentation: zram: update IDLE pages tracking documentation

Move IDLE pages tracking into a separate chapter because there
are multiple features that use (or depend on) it either in
built-in variant ("mark all") or in extended variant (ac-time
tracking).

In addition, recompression doesn't require memory tracking
to be enabled in order to be able to perform idle recompression.

Reported-by: Shin Kawamura <kawasin@...gle.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
---
 Documentation/admin-guide/blockdev/zram.rst | 41 +++++++++++----------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index b8d36134a151..3e273c1bb749 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -317,6 +317,26 @@ a single line of text and contains the following stats separated by whitespace:
 Optional Feature
 ================
 
+IDLE pages tracking
+-------------------
+
+zram has built-in support for idle pages tracking (that is, allocated but
+not used pages). This feature is useful for e.g. zram writeback and
+recompression. In order to mark pages as idle, execute the following command::
+
+	echo all > /sys/block/zramX/idle
+
+This will mark all allocated zram pages as idle. The idle mark will be
+removed only when the page (block) is accessed (e.g. overwritten or freed).
+Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled, pages can be
+marked as idle based on how many seconds have passed since the last access to
+a particular zram page::
+
+	echo 86400 > /sys/block/zramX/idle
+
+In this example, all pages which haven't been accessed in more than 86400
+seconds (one day) will be marked idle.
+
 writeback
 ---------
 
@@ -331,24 +351,7 @@ If admin wants to use incompressible page writeback, they could do it via::
 
 	echo huge > /sys/block/zramX/writeback
 
-To use idle page writeback, first, user need to declare zram pages
-as idle::
-
-	echo all > /sys/block/zramX/idle
-
-From now on, any pages on zram are idle pages. The idle mark
-will be removed until someone requests access of the block.
-IOW, unless there is access request, those pages are still idle pages.
-Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled pages can be
-marked as idle based on how long (in seconds) it's been since they were
-last accessed::
-
-        echo 86400 > /sys/block/zramX/idle
-
-In this example all pages which haven't been accessed in more than 86400
-seconds (one day) will be marked idle.
-
-Admin can request writeback of those idle pages at right timing via::
+Admin can request writeback of idle pages at right timing via::
 
 	echo idle > /sys/block/zramX/writeback
 
@@ -499,8 +502,6 @@ attempt to recompress:::
 
 	echo "type=huge_idle max_pages=42" > /sys/block/zramX/recompress
 
-Recompression of idle pages requires memory tracking.
-
 During re-compression for every page, that matches re-compression criteria,
 ZRAM iterates the list of registered alternative compression algorithms in
 order of their priorities. ZRAM stops either when re-compression was
-- 
2.49.0.604.gff1f9ca942-goog