Re: [PATCH v4 1/2] mm: zswap: fix global shrinker memcg iteration

[Nhat Pham <nphamcs@gmail.com>]

On Sat, Jul 27, 2024 at 4:06 PM Takero Funaki <flintglass@gmail.com> wrote:
>
> This patch fixes an issue where the zswap global shrinker stopped
> iterating through the memcg tree.
>
> The problem was that shrink_worker() would restart iterating memcg tree
> from the tree root, considering an offline memcg as a failure, and abort
> shrinking after encountering the same offline memcg 16 times even if
> there is only one offline memcg. After this change, an offline memcg in
> the tree is no longer considered a failure. This allows the shrinker to
> continue shrinking the other online memcgs regardless of whether an
> offline memcg exists, gives higher zswap writeback activity.
>
> To avoid holding refcount of offline memcg encountered during the memcg
> tree walking, shrink_worker() must continue iterating to release the
> offline memcg to ensure the next memcg stored in the cursor is online.
>
> The offline memcg cleaner has also been changed to avoid the same issue.
> When the next memcg of the offlined memcg is also offline, the refcount
> stored in the iteration cursor was held until the next shrink_worker()
> run. The cleaner must release the offline memcg recursively.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@gmail.com>
> ---
>  mm/zswap.c | 73 ++++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 49 insertions(+), 24 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index adeaf9c97fde..e9b5343256cd 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -765,12 +765,31 @@ void zswap_folio_swapin(struct folio *folio)
>         }
>  }
>
> +/*
> + * This function should be called when a memcg is being offlined.
> + *
> + * Since the global shrinker shrink_worker() may hold a reference
> + * of the memcg, we must check and release the reference in
> + * zswap_next_shrink.
> + *
> + * shrink_worker() must handle the case where this function releases
> + * the reference of memcg being shrunk.
> + */
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  {
>         /* lock out zswap shrinker walking memcg tree */
>         spin_lock(&zswap_shrink_lock);
> -       if (zswap_next_shrink == memcg)
> -               zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +       if (zswap_next_shrink == memcg) {
> +               do {
> +                       zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +               } while (zswap_next_shrink && !mem_cgroup_online(zswap_next_shrink));
> +               /*
> +                * We verified the next memcg is online.  Even if the next
> +                * memcg is being offlined here, another cleaner must be
> +                * waiting for our lock.  We can leave the online memcg
> +                * reference.
> +                */
> +       }
>         spin_unlock(&zswap_shrink_lock);
>  }
>
> @@ -1304,43 +1323,49 @@ static void shrink_worker(struct work_struct *w)
>         /* Reclaim down to the accept threshold */
>         thr = zswap_accept_thr_pages();
>
> -       /* global reclaim will select cgroup in a round-robin fashion. */
> +       /* global reclaim will select cgroup in a round-robin fashion.
> +        *
> +        * We save iteration cursor memcg into zswap_next_shrink,
> +        * which can be modified by the offline memcg cleaner
> +        * zswap_memcg_offline_cleanup().
> +        *
> +        * Since the offline cleaner is called only once, we cannot leave an
> +        * offline memcg reference in zswap_next_shrink.
> +        * We can rely on the cleaner only if we get online memcg under lock.
> +        *
> +        * If we get an offline memcg, we cannot determine if the cleaner has
> +        * already been called or will be called later. We must put back the
> +        * reference before returning from this function. Otherwise, the
> +        * offline memcg left in zswap_next_shrink will hold the reference
> +        * until the next run of shrink_worker().
> +        */
>         do {
>                 spin_lock(&zswap_shrink_lock);
> -               zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> -               memcg = zswap_next_shrink;
>
>                 /*
> -                * We need to retry if we have gone through a full round trip, or if we
> -                * got an offline memcg (or else we risk undoing the effect of the
> -                * zswap memcg offlining cleanup callback). This is not catastrophic
> -                * per se, but it will keep the now offlined memcg hostage for a while.
> -                *
> +                * Start shrinking from the next memcg after zswap_next_shrink.
> +                * When the offline cleaner has already advanced the cursor,
> +                * advancing the cursor here overlooks one memcg, but this
> +                * should be negligibly rare.
> +                */
> +               do {
> +                       memcg = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +                       zswap_next_shrink = memcg;
> +               } while (memcg && !mem_cgroup_tryget_online(memcg));
> +               /*

Yeah I agree with Yosry's comment - the do while loop looks like it
can become a helper in some form? But that asides, the rest LGTM:

Reviewed-by: Nhat Pham <nphamcs@gmail.com>