Re: [PATCH v2 3/3] mm/zswap: preserve the workingset shadow across writeback

[Alexandre Ghiti]

Hi David,

On Fri, Aug 21, 2026 at 11:58 AM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> >
> On 8/21/26 11:35, Alexandre Ghiti wrote:
> > When zswap writes an entry back, allocating the buffer folio in the swap
> > cache overwrites the swap slot's workingset shadow. It is lost and later
> > re-minted inaccurately, which corrupts anon refault/workingset accounting
> > for the written-back data.
> >
> > Preserve it within zswap itself: at writeback, park the shadow in the
> > zswap tree in place of the freed entry, so it outlives the writeback
> > buffer folio. The buffer then meets one of three fates, and the parked
> > shadow is handled at whichever happens first:
> >
> >  - swap-in: do_swap_page()/shmem_swapin_folio() find the buffer in the swap
> >    cache, take the shadow and feed it to workingset_refault() to account
> >    the refault of the now-resident folio, then drop it.
> >  - reclaim: __remove_mapping() skips minting a fresh shadow for the buffer
> >    (minting would double-count the eviction) and
> >    __swap_cache_do_del_folio() restores the parked shadow into the slot, so
> >    a later disk swap-in still refaults against the original eviction.
> >  - slot free (e.g. process exit): the shadow is not needed and is cleared.
> >
> > The last two both go through __swap_cache_do_del_folio(), the single point
> > every swap-cache removal passes through, which is why the shadow is read
> > and cleared there.
> >
> > Fixes: aae466b0052e ("mm/swap: implement workingset detection for anonymous LRU")
> > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> >  include/linux/swap.h  |  2 +-
> >  include/linux/zswap.h | 12 ++++++
> >  mm/filemap.c          |  2 +-
> >  mm/memory.c           |  5 +++
> >  mm/shmem.c            |  5 +++
> >  mm/swap_state.c       | 14 ++++++-
> >  mm/vmscan.c           |  4 +-
> >  mm/workingset.c       | 16 ++++++--
> >  mm/zswap.c            | 91 ++++++++++++++++++++++++++++++++++++++++++-
> >  9 files changed, 142 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 6d72778e6cc3..ef9b1612ac12 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -298,7 +298,7 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset,
> >                               bool flush);
> >  void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
> >  void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);
> > -void workingset_refault(struct folio *folio, void *shadow);
> > +void workingset_refault(struct folio *folio, void *shadow, bool lru_managed);
>
> Passing booleans makes calling code hard to read. It's preferable to either use
>
> (a) Dedicated flags; only makes sense if there are really more flags expected
>
> (b) Having helpfer functions that spell it out for calling code
>
> workingset_refault_lru_managed
> workingset_refault
>
> workingset_refault_lru
> workingset_refault_nonlru
>
> Or something like that, which makes calling code easier to get.

Ok, I'll see which of (a) or (b) is less intrusive.

Thanks,

Alex

>
> --
> Cheers,
>
> David
>