Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration

[Yosry Ahmed]

On Sat, Jun 8, 2024 at 8:53 AM Takero Funaki <flintglass@xxxxxxxxx> wrote:
>
> This patch fixes an issue where the zswap global shrinker stopped
> iterating through the memcg tree.
>
> The problem was that `shrink_worker()` would stop iterating when a memcg
> was being offlined and restart from the tree root.  Now, it properly
> handles the offlining memcg and continues shrinking with the next memcg.
>
> This patch also modified handing of the lock for offlined memcg cleaner
> to adapt the change in the iteration, and avoid negligibly rare skipping
> of a memcg from shrink iteration.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@xxxxxxxxx>
> ---
>  mm/zswap.c | 87 ++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 68 insertions(+), 19 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 80c634acb8d5..d720a42069b6 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -827,12 +827,27 @@ 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 (READ_ONCE(zswap_next_shrink) == memcg) {
> +               /* put back reference and advance the cursor */
> +               memcg = mem_cgroup_iter(NULL, memcg, NULL);
> +               WRITE_ONCE(zswap_next_shrink, memcg);
> +       }
> +

I am really finding it difficult to understand what the diff is trying
to do. We are holding a lock that protects zswap_next_shrink. We
always access it with the lock held. Why do we need all of this?

Adding READ_ONCE() and WRITE_ONCE() where they are not needed is just
confusing imo.

>         spin_unlock(&zswap_shrink_lock);
>  }
>
> @@ -1401,25 +1416,44 @@ static int shrink_memcg(struct mem_cgroup *memcg)
>
>  static void shrink_worker(struct work_struct *w)
>  {
> -       struct mem_cgroup *memcg;
> +       struct mem_cgroup *memcg = NULL;
> +       struct mem_cgroup *next_memcg;
>         int ret, failures = 0;
>         unsigned long thr;
>
>         /* 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 abandone
> +        * offline memcg reference in zswap_next_shrink.
> +        * We can rely on the cleaner only if we get online memcg under lock.
> +        * If we get offline memcg, we cannot determine the cleaner will be
> +        * called later. We must put it before returning from this function.
> +        */
>         do {
> +iternext:
>                 spin_lock(&zswap_shrink_lock);
> -               zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> -               memcg = zswap_next_shrink;
> +               next_memcg = READ_ONCE(zswap_next_shrink);
> +
> +               if (memcg != next_memcg) {
> +                       /*
> +                        * Ours was released by offlining.
> +                        * Use the saved memcg reference.
> +                        */
> +                       memcg = next_memcg;

'memcg' will always be NULL on the first iteration, so we will always
start by shrinking 'zswap_next_shrink' for a second time before moving
the iterator.

> +               } else {
> +                       /* advance cursor */
> +                       memcg = mem_cgroup_iter(NULL, memcg, NULL);
> +                       WRITE_ONCE(zswap_next_shrink, memcg);

Again, I don't see what this is achieving. The first iteration will
always set 'memcg' to 'zswap_next_shrink', and then we will always
move the iterator forward. The only difference I see is that we shrink
'zswap_next_shrink' twice in a row now (last 'memcg' in prev call, and
first 'memcg' in this call).

> +               }
>
>                 /*
> -                * 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.
> -                *
>                  * Note that if we got an online memcg, we will keep the extra
>                  * reference in case the original reference obtained by mem_cgroup_iter
>                  * is dropped by the zswap memcg offlining callback, ensuring that the
> @@ -1434,16 +1468,25 @@ static void shrink_worker(struct work_struct *w)
>                 }
>
>                 if (!mem_cgroup_tryget_online(memcg)) {
> -                       /* drop the reference from mem_cgroup_iter() */
> -                       mem_cgroup_iter_break(NULL, memcg);
> -                       zswap_next_shrink = NULL;
> +                       /*
> +                        * It is an offline memcg which we cannot shrink
> +                        * until its pages are reparented.
> +                        *
> +                        * Since we cannot determine if the offline cleaner has
> +                        * been already called or not, the offline memcg must be
> +                        * put back unconditonally. We cannot abort the loop while
> +                        * zswap_next_shrink has a reference of this offline memcg.
> +                        */

You actually deleted the code that actually puts the ref to the
offline memcg above.

Why don't you just replace mem_cgroup_iter_break(NULL, memcg) with
mem_cgroup_iter(NULL, memcg, NULL) here? I don't understand what the
patch is trying to do to be honest. This patch is a lot more confusing
than it should be.

Also, I would like Nhat to weigh in here. Perhaps the decision to
reset the iterator instead of advancing it in this case was made for a
reason that we should honor. Maybe cgroups are usually offlined
together so we will keep running into offline cgroups here if we
continue? I am not sure.

>                         spin_unlock(&zswap_shrink_lock);
> -
> -                       if (++failures == MAX_RECLAIM_RETRIES)
> -                               break;
> -
> -                       goto resched;
> +                       goto iternext;
>                 }
> +               /*
> +                * We got an extra memcg reference before unlocking.
> +                * The cleaner cannot free it using zswap_next_shrink.
> +                *
> +                * Our memcg can be offlined after we get online memcg here.
> +                * In this case, the cleaner is waiting the lock just behind us.
> +                */
>                 spin_unlock(&zswap_shrink_lock);
>
>                 ret = shrink_memcg(memcg);
> @@ -1457,6 +1500,12 @@ static void shrink_worker(struct work_struct *w)
>  resched:
>                 cond_resched();
>         } while (zswap_total_pages() > thr);
> +
> +       /*
> +        * We can still hold the original memcg reference.
> +        * The reference is stored in zswap_next_shrink, and then reused
> +        * by the next shrink_worker().
> +        */
>  }
>
>  /*********************************
> --
> 2.43.0
>