[RFC PATCH v2 0/7] mm: zswap: move writeback LRU from zpool to zswap

Domenico Cerasuolo
2023-06-06T14:57:47Z

This series aims to improve the zswap reclaim mechanism by reorganizing
the LRU management. In the current implementation, the LRU is maintained
within each zpool driver, resulting in duplicated code across the three
drivers. The proposed change consists in moving the LRU management from
the individual implementations up to the zswap layer.

The primary objective of this refactoring effort is to simplify the
codebase. By unifying the reclaim loop and consolidating LRU handling
within zswap, we can eliminate redundant code and improve
maintainability. Additionally, this change enables the reclamation of
stored pages in their actual LRU order. Presently, the zpool drivers
link backing pages in an LRU, causing compressed pages with different
LRU positions to be written back simultaneously.

The series consists of several patches. The first patch implements the
LRU and the reclaim loop in zswap, but it is not used yet because all
three driver implementations are marked as zpool_evictable.
The following three commits modify each zpool driver to be not
zpool_evictable, allowing the use of the reclaim loop in zswap.
As the drivers removed their shrink functions, the zpool interface is
then trimmed by removing zpool_evictable, zpool_ops, and zpool_shrink.
Finally, the code in zswap is further cleaned up by simplifying the
writeback function and removing the now unnecessary zswap_header.

Based on mm-stable + commit 399ab221f3ff
("mm: zswap: shrink until can accept") currently in mm-unstable.

V2:
- fixed lru list init/del/del_init (Johannes)
- renamed pool.lock to lru_lock and added lock ordering comment (Yosry)
- trimmed zsmalloc even more (Johannes | Nhat)
- moved ref drop out of writeback function (Johannes)

Domenico Cerasuolo (7):
mm: zswap: add pool shrinking mechanism
mm: zswap: remove page reclaim logic from zbud
mm: zswap: remove page reclaim logic from z3fold
mm: zswap: remove page reclaim logic from zsmalloc
mm: zswap: remove shrink from zpool interface
mm: zswap: simplify writeback function
mm: zswap: remove zswap_header

include/linux/zpool.h | 19 +-
mm/z3fold.c | 249 +-------------------------
mm/zbud.c | 167 +-----------------
mm/zpool.c | 48 +----
mm/zsmalloc.c | 396 ++----------------------------------------
mm/zswap.c | 186 +++++++++++---------
6 files changed, 130 insertions(+), 935 deletions(-)

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2.34.1