[RFC PATCH v1 0/5] Introduce objects folding mechanism

[Alexey Romanov]

Hello!

This RFC series adds feature which allows fold identical
zsmalloc objects into a single one.

Based on ZRAM version:
https://lore.kernel.org/lkml/Y3w8%2Fq%2FHoSbqamoD@xxxxxxxxxx/t/

Let's imagine that 3 objects with the same content got into zsmalloc:

+----------------+   +----------------+   +----------------+
|    handle 1    |   |    handle 2    |   |    handle 3    |
+-------+--------+   +-------+--------+   +--------+-------+
        |                    |                     |
        |                    |                     |
+-------v--------+   +-------v---------+  +--------v-------+
|zsmalloc  object|   |zsmalloc  object |  |zsmalloc  object|
++--------------++   +-+-------------+-+  ++--------------++
 +--------------+      +-------------+     +--------------+
 | buffer: "abc"|      |buffer: "abc"|     | buffer: "abc"|
 +--------------+      +-------------+     +--------------+

As you can see, the data is duplicated. Fold mechanism saves
(after scanning objects) only one zsmalloc object. Here's
what happens after the scan and fold:

+----------------+   +----------------+   +----------------+
|    handle 1    |   |    handle 2    |   |    handle 3    |
+-------+--------+   +-------+--------+   +--------+-------+
        |                    |                     |
        |                    |                     |
        |           +--------v---------+           |
        +-----------> zsmalloc  object <-----------+
                    +--+-------------+-+
                       +-------------+
                       |buffer: "abc"|
                       +-------------+

Thus, we reduced the amount of memory occupied by 3 times.

This mechanism doesn't affect the perf of the zsmalloc itself in
any way (maybe just a little bit on the zs_free() function).
In order to describe each such identical object, we (constantly)
need sizeof(fold_rbtree_node) bytes. Also, all struct size_class now
have new field struct rb_root fold_rbtree.

Testing on my system (8GB RAM + 1Gb ZRAM SWAP) showed that at high
loads, on average, when calling the fold mechanism, we can save
up to 15-20% of the memory usage.

This patch series adds a new sysfs node into ZRAM - trigger folding
and provides new field in mm_stat. This field shows how many pages
freed during folding:

  $ cat /sys/block/zram0/mm_stat
    431452160 332984392 339894272 0 339894272 282 0 51374 51374 0

  $ echo 1 > /sys/block/zram0/fold

  $ cat /sys/block/zram/mm_stat
    431452160 270376848 287301504 0 339894272 282 0 51374 51374 6593

Alexey Romanov (5):
  mm/zsmalloc: use ARRAY_SIZE in isolate_zspage()
  mm/zsmalloc: get rid of PAGE_MASK
  mm/zsmalloc: introduce objects folding mechanism
  zram: add fold sysfs knob
  zram: add pages_folded to stats

 Documentation/admin-guide/blockdev/zram.rst |   2 +
 drivers/block/zram/zram_drv.c               |  30 +-
 include/linux/zsmalloc.h                    |   4 +
 mm/Kconfig                                  |   9 +
 mm/zsmalloc.c                               | 484 +++++++++++++++++++-
 5 files changed, 513 insertions(+), 16 deletions(-)

-- 
2.38.1