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zram: modernize writeback interface

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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>
This commit is contained in:
Sergey Senozhatsky
2025-03-27 10:58:09 +09:00
committed by Andrew Morton
parent 0bf19a357e
commit cf42d4cccf
2 changed files with 233 additions and 106 deletions
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17
Documentation/admin-guide/blockdev/zram.rst
View File

@@ -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.