zram: modernize writeback interface

The writeback interface supports a page_index=N parameter which performs
writeback of the given page.  Since we rarely need to writeback just one
single page, the typical use case involves a number of writeback calls,
each performing writeback of one page:

  echo page_index=100 > zram0/writeback
  ...
  echo page_index=200 > zram0/writeback
  echo page_index=500 > zram0/writeback
  ...
  echo page_index=700 > zram0/writeback

One obvious downside of this is that it increases the number of syscalls. 
Less obvious, but a significantly more important downside, is that when
given only one page to post-process zram cannot perform an optimal target
selection.  This becomes a critical limitation when writeback_limit is
enabled, because under writeback_limit we want to guarantee the highest
memory savings hence we first need to writeback pages that release the
highest amount of zsmalloc pool memory.

This patch adds page_indexes=LOW-HIGH parameter to the writeback
interface:

  echo page_indexes=100-200 page_indexes=500-700 > zram0/writeback

This gives zram a chance to apply an optimal target selection strategy on
each iteration of the writeback loop.

We also now permit multiple page_index parameters per call (previously
zram would recognize only one page_index) and a mix or single pages and
page ranges:

  echo page_index=42 page_index=99 page_indexes=100-200 \
       page_indexes=500-700 > zram0/writeback

Apart from that the patch also unifies parameters passing and resembles
other "modern" zram device attributes (e.g.  recompression), while the old
interface used a mixed scheme: values-less parameters for mode and a
key=value format for page_index.  We still support the "old" value-less
format for compatibility reasons.

[senozhatsky@chromium.org: simplify parse_page_index() range checks, per Brian]
  nk: https://lkml.kernel.org/r/20250404015327.2427684-1-senozhatsky@chromium.org
[sozhatsky@chromium.org: fix uninitialized variable in zram_writeback_slots(), per Dan]
  nk: https://lkml.kernel.org/r/20250409112611.1154282-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20250327015818.4148660-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 9bdb30901a93..b8d36134a151 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -369,6 +369,23 @@ they could write a page index into the interface::
 
 	echo "page_index=1251" > /sys/block/zramX/writeback
 
+In Linux 6.16 this interface underwent some rework.  First, the interface
+now supports `key=value` format for all of its parameters (`type=huge_idle`,
+etc.)  Second, the support for `page_indexes` was introduced, which specify
+`LOW-HIGH` range (or ranges) of pages to be written-back.  This reduces the
+number of syscalls, but more importantly this enables optimal post-processing
+target selection strategy. Usage example::
+
+	echo "type=idle" > /sys/block/zramX/writeback
+	echo "page_indexes=1-100 page_indexes=200-300" > \
+		/sys/block/zramX/writeback
+
+We also now permit multiple page_index params per call and a mix of
+single pages and page ranges::
+
+	echo page_index=42 page_index=99 page_indexes=100-200 \
+		page_indexes=500-700 > /sys/block/zramX/writeback
+
 If there are lots of write IO with flash device, potentially, it has
 flash wearout problem so that admin needs to design write limitation
 to guarantee storage health for entire product life.