Re: [PATCH v2 9/9] mm: zswap: per-node kmem accounting for zswap/zsmalloc

[Nhat Pham]

On Fri, Jun 26, 2026 at 7:32 AM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
> On Fri, 26 Jun 2026 12:20:58 +0200 Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> > Update zswap and zsmalloc to use per-node obj_cgroup for kmem
> > accounting, attributing compressed page charges to the correct
> > NUMA node.
> >
> > But actually, this is incomplete because it does not correctly account
> > for entries that straddle pages, those pages being possibly on 2 different
> > nodes.
> >
> > This will be correctly handled by Joshua in a different series [1].
> >
> > Link: https://lore.kernel.org/linux-mm/20260311195153.4013476-1-joshua.hahnjy@xxxxxxxxx/ [1]
> > Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> >  include/linux/zsmalloc.h |  2 ++
> >  mm/zsmalloc.c            | 11 +++++++++++
> >  mm/zswap.c               | 19 ++++++++++++++++++-
> >  3 files changed, 31 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
> > index 478410c880b1..30427f3fe232 100644
> > --- a/include/linux/zsmalloc.h
> > +++ b/include/linux/zsmalloc.h
> > @@ -50,6 +50,8 @@ void zs_obj_read_sg_end(struct zs_pool *pool, unsigned long handle);
> >  void zs_obj_write(struct zs_pool *pool, unsigned long handle,
> >                 void *handle_mem, size_t mem_len);
> >
> > +int zs_handle_to_nid(struct zs_pool *pool, unsigned long handle);
> > +
> >  extern const struct movable_operations zsmalloc_mops;
> >
> >  #endif
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 83f5820c45f9..17f7403ebe77 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1380,6 +1380,17 @@ static void obj_free(int class_size, unsigned long obj)
> >       mod_zspage_inuse(zspage, -1);
> >  }
> >
> > +int zs_handle_to_nid(struct zs_pool *pool, unsigned long handle)
> > +{
> > +     unsigned long obj;
> > +     struct zpdesc *zpdesc;
> > +
> > +     obj = handle_to_obj(handle);
> > +     obj_to_zpdesc(obj, &zpdesc);
> > +     return page_to_nid(zpdesc_page(zpdesc));
> > +}
> > +EXPORT_SYMBOL(zs_handle_to_nid);
>
> Does this need the same locking as the other handle-to-zspage paths?
> zs_free() takes pool->lock before handle_to_obj() because zspage migration can
> update or move the object behind the handle. This helper does the same decode
> without the lock, so zswap's uncharge path can race migration and charge or
> uncharge the wrong node, or observe transient zspage state.
>

Can we just charge it to the page's node for now? Once Joshua's patch
series is in, we can correctly charge the node owning the data :)

FWIW, this is how these zswap entries are organized in the LRU too -
following to the OG page's node.