linux-kernel - Re: [PATCH v3 1/2] mm: zswap: fix global shrinker memcg iteration

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Message-ID: <CAKEwX=NGu_MM3bzT9eXAAJhvCvv+x4Qvf77=_RFD-M7zxKFriA@mail.gmail.com>
Date: Mon, 22 Jul 2024 14:39:23 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Takero Funaki <flintglass@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: zswap: fix global shrinker memcg iteration

On Fri, Jul 19, 2024 at 9:41 PM Takero Funaki <flintglass@...il.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 stop iterating when a memcg
> was being offlined and restart from the tree root.  Now, it properly
> handles the offline memcg and continues shrinking with the next memcg.
>
> 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@...il.com>
Hmm LGTM for the most part - a couple nits
[...]
> +                       zswap_next_shrink = mem_cgroup_iter(NULL,
> +                                       zswap_next_shrink, NULL);
nit: this can fit in a single line right? Looks like it's exactly 80 characters.
[...]
> +                       zswap_next_shrink = mem_cgroup_iter(NULL,
> +                                               zswap_next_shrink, NULL);
Same with this.
[...]
> +               /*
> +                * We verified the memcg is online and got an extra memcg
> +                * reference.  Our memcg might be offlined concurrently but the
> +                * respective offline cleaner must be waiting for our lock.
> +                */
>                 spin_unlock(&zswap_shrink_lock);
nit: can we remove this spin_unlock() call + the one within the `if
(!memcg)` block, and just do it unconditionally outside of if
(!memcg)? Looks like we are unlocking regardless of whether memcg is
null or not.

memcg is a local variable, not protected by zswap_shrink_lock, so this
should be fine right?

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