Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Nhat Pham]

On Mon, Feb 12, 2024 at 5:29 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> On 2024/2/12 05:21, Nhat Pham wrote:
> > On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> > <zhouchengming@xxxxxxxxxxxxx> wrote:
> >>
> >> All zswap entries will take a reference of zswap_pool when
> >> zswap_store(), and drop it when free. Change it to use the
> >> percpu_ref is better for scalability performance.
> >>
> >> Testing kernel build in tmpfs with memory.max=2GB
> >> (zswap shrinker and writeback enabled with one 50GB swapfile).
> >>
> >>         mm-unstable  zswap-global-lru
> >> real    63.20        63.12
> >> user    1061.75      1062.95
> >> sys     268.74       264.44
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> >> ---
> >>  mm/zswap.c | 30 +++++++++++++++++++++---------
> >>  1 file changed, 21 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 7668db8c10e3..afb31904fb08 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
> >>  struct zswap_pool {
> >>         struct zpool *zpools[ZSWAP_NR_ZPOOLS];
> >>         struct crypto_acomp_ctx __percpu *acomp_ctx;
> >> -       struct kref kref;
> >> +       struct percpu_ref ref;
> >>         struct list_head list;
> >>         struct work_struct release_work;
> >>         struct hlist_node node;
> >> @@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
> >>  /*********************************
> >>  * pool functions
> >>  **********************************/
> >> +static void __zswap_pool_empty(struct percpu_ref *ref);
> >>
> >>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> >>  {
> >> @@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> >>         /* being the current pool takes 1 ref; this func expects the
> >>          * caller to always add the new pool as the current pool
> >>          */
> >> -       kref_init(&pool->kref);
> >> +       ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
> >> +                             PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
> >> +       if (ret)
> >> +               goto ref_fail;
> >>         INIT_LIST_HEAD(&pool->list);
> >>
> >>         zswap_pool_debug("created", pool);
> >>
> >>         return pool;
> >>
> >> +ref_fail:
> >> +       cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
> >>  error:
> >>         if (pool->acomp_ctx)
> >>                 free_percpu(pool->acomp_ctx);
> >> @@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
> >>
> >>         synchronize_rcu();
> >>
> >> -       /* nobody should have been able to get a kref... */
> >> -       WARN_ON(kref_get_unless_zero(&pool->kref));
> >
> > Do we no longer care about this WARN? IIUC, this is to catch someone
> > still holding a reference to the pool at release time, which sounds
> > like a bug. I think we can simulate the similar behavior with:
>
> Ok, I thought it has already been put to 0 when we're here, so any tryget
> will fail. But keeping this WARN_ON() is also fine to me, will keep it.

Yup - it should fail, if the code is not buggy. But that's a pretty big if :)

Jokes aside, we can remove it if folks think the benefit is not worth
the cost/overhead. However, I'm a bit hesitant to remove checks in
zswap, especially given how buggy it has been (some of which are
refcnt bugs as well, IIRC).