[PATCH v3 00/11] zsmalloc: redesign object alloc API

[Sergey Senozhatsky]

Both zram and zswap already have per-entry structures that store zsmalloc
handles, so allocating a separate handle_cachep object per stored page is
redundant.  This series embeds struct zs_handle directly into zram and
zswap entries and drops handle_cachep, saving 8 bytes and a slab
alloc/free per stored object (2MB per 1GB of swapped out 4K pages).

It also folds zs_malloc() and zs_obj_write() into zs_store() so we don't
take pool->lock and look up object location again right after allocation.

A simple zram test on a 24-vCPU VM
==================================
165 fewer kernel instructions and 0.7-1.8% fewer kernel cycles per write;
lz4 single writer +2.3% IOPS / -2.6% average latency;
24 writers +0.9-+1.0% IOPS; no regressions.
Memory: 8 bytes less per stored page.

v2 -> v3:
- addressed Sashiko review: alignment and READ/WRITE_ONCE mem_limit
- picked up review tags from Yosry and Nhat
- added handle validation helper
- change definition of ZS_HANDLE_SIZE

Sergey Senozhatsky (11):
  zram: drop stale comment in write_incompressible_page()
  zram: drop get_slot_handle() and set_slot_handle()
  zsmalloc: return handle from obj_allocated()
  zsmalloc: switch handle type to struct zs_handle
  zram: check mem_limit before allocating zsmalloc object
  zram: allocate zsmalloc object under slot lock
  zswap: do not free the entry if its pool is NULL
  zsmalloc: encode 1-based obj_idx so valid obj is never 0
  zsmalloc: embed struct zs_handle into zram and zswap entries
  zsmalloc: fold zs_obj_write() into zs_store()
  zsmalloc: populate new zspage outside class->lock in zs_store()

 Documentation/admin-guide/blockdev/zram.rst  |   4 +-
 Documentation/admin-guide/mm/slab.rst        |   2 +-
 Documentation/core-api/memory-allocation.rst |   2 +-
 Documentation/mm/zsmalloc.rst                |  22 +-
 drivers/block/zram/zram_drv.c                | 148 ++++------
 drivers/block/zram/zram_drv.h                |  10 +-
 include/linux/zsmalloc.h                     |  36 ++-
 mm/zsmalloc.c                                | 288 +++++++++++--------
 mm/zswap.c                                   |  28 +-
 9 files changed, 297 insertions(+), 243 deletions(-)

-- 
2.56.0.542.g3ef89b7020-goog