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

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Jun 6, 2023 at 7:56 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> 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.

I tested this + commit fe1d1f7d0fb5 ("mm: zswap: support exclusive
loads") currently in mm-unstable, using zsmalloc and
CONFIG_ZSWAP_EXCLUSIVE_LOADS=y. I only ran basic zswap tests with
manual writeback induction and made sure everything is sane. I
obviously hope you did more involved testing :)

The only problem I came across is the conflict with fe1d1f7d0fb5, and
I suggested the fix in patch 1. With the fix, everything seems
correct.

So I guess, FWIW for all the patches except 2 & 3 (for zbud and z3fold):
Tested-by: Yosry Ahmed <yosryahmed@google.com>

>
> 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(-)
>
> --
> 2.34.1
>