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

[Domenico Cerasuolo <cerasuolodomenico@gmail.com>]

On Wed, Jun 7, 2023 at 11:16 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> 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>

Thanks a lot for the effort! I'll rebase and test it again before submitting the
new version.

>
> >
> > 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
> >