Re: [PATCH v1 2/3] mm: zswap: fix global shrinker error handling logic

[Yosry Ahmed <yosryahmed@google.com>]

On Sat, Jun 8, 2024 at 8:53 AM Takero Funaki <flintglass@gmail.com> wrote:
>
> This patch fixes zswap global shrinker that did not shrink zpool as
> expected.
>
> The issue it addresses is that `shrink_worker()` did not distinguish
> between unexpected errors and expected error codes that should be
> skipped, such as when there is no stored page in a memcg. This led to
> the shrinking process being aborted on the expected error codes.
>
> The shrinker should ignore these cases and skip to the next memcg.
> However,  skipping all memcgs presents another problem. To address this,
> this patch tracks progress while walking the memcg tree and checks for
> progress once the tree walk is completed.
>
> To handle the empty memcg case, the helper function `shrink_memcg()` is
> modified to check if the memcg is empty and then return -ENOENT.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@gmail.com>
> ---
>  mm/zswap.c | 31 ++++++++++++++++++++++++++-----
>  1 file changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index d720a42069b6..1a90f434f247 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1393,7 +1393,7 @@ static struct shrinker *zswap_alloc_shrinker(void)
>
>  static int shrink_memcg(struct mem_cgroup *memcg)
>  {
> -       int nid, shrunk = 0;
> +       int nid, shrunk = 0, stored = 0;
>
>         if (!mem_cgroup_zswap_writeback_enabled(memcg))
>                 return -EINVAL;
> @@ -1408,9 +1408,16 @@ static int shrink_memcg(struct mem_cgroup *memcg)
>         for_each_node_state(nid, N_NORMAL_MEMORY) {
>                 unsigned long nr_to_walk = 1;
>
> +               if (!list_lru_count_one(&zswap_list_lru, nid, memcg))
> +                       continue;
> +               ++stored;
>                 shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
>                                             &shrink_memcg_cb, NULL, &nr_to_walk);
>         }
> +
> +       if (!stored)
> +               return -ENOENT;
> +

Can't we just check nr_to_walk here and return -ENOENT if it remains as 1?

Something like:

if (nr_to_walk)
    return -ENOENT;
if (!shrunk)
    return -EAGAIN;
return 0;

>         return shrunk ? 0 : -EAGAIN;
>  }
>
> @@ -1418,12 +1425,18 @@ static void shrink_worker(struct work_struct *w)
>  {
>         struct mem_cgroup *memcg = NULL;
>         struct mem_cgroup *next_memcg;
> -       int ret, failures = 0;
> +       int ret, failures = 0, progress;
>         unsigned long thr;
>
>         /* Reclaim down to the accept threshold */
>         thr = zswap_accept_thr_pages();
>
> +       /*
> +        * We might start from the last memcg.
> +        * That is not a failure.
> +        */
> +       progress = 1;
> +
>         /* global reclaim will select cgroup in a round-robin fashion.
>          *
>          * We save iteration cursor memcg into zswap_next_shrink,
> @@ -1461,9 +1474,12 @@ static void shrink_worker(struct work_struct *w)
>                  */
>                 if (!memcg) {
>                         spin_unlock(&zswap_shrink_lock);
> -                       if (++failures == MAX_RECLAIM_RETRIES)
> +
> +                       /* tree walk completed but no progress */
> +                       if (!progress && ++failures == MAX_RECLAIM_RETRIES)
>                                 break;

It seems like we may keep iterating the entire hierarchy a lot of
times as long as we are making any type of progress. This doesn't seem
right.

>
> +                       progress = 0;
>                         goto resched;
>                 }
>
> @@ -1493,10 +1509,15 @@ static void shrink_worker(struct work_struct *w)
>                 /* drop the extra reference */
>                 mem_cgroup_put(memcg);
>
> -               if (ret == -EINVAL)
> -                       break;
> +               /* not a writeback candidate memcg */
> +               if (ret == -EINVAL || ret == -ENOENT)
> +                       continue;
> +

We should probably return -ENOENT for memcg with writeback disabled as well.

>                 if (ret && ++failures == MAX_RECLAIM_RETRIES)
>                         break;
> +
> +               ++progress;
> +               /* reschedule as we performed some IO */
>  resched:
>                 cond_resched();
>         } while (zswap_total_pages() > thr);
> --
> 2.43.0
>