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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 9 May 2023 10:55:05 +0200
Message-ID: <CANn89iK154B-NzRFymx_ggO9ZuVW-0YyHEKi6C46zjHpdRfokQ@mail.gmail.com>
Subject: Re: [PATCH] sock: Fix misuse of sk_under_memory_pressure()
To:     Abel Wu <wuyun.abel@bytedance.com>
Cc:     "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Sat, May 6, 2023 at 10:59=E2=80=AFAM Abel Wu <wuyun.abel@bytedance.com> =
wrote:
>
> The commit 180d8cd942ce ("foundations of per-cgroup memory pressure
> controlling") wrapped proto::memory_pressure status into an accessor
> named sk_under_memory_pressure(), and in the next commit e1aab161e013
> ("socket: initial cgroup code") added the consideration of net-memcg
> pressure into this accessor.
>
> But with the former patch applied, not all of the call sites of
> sk_under_memory_pressure() are interested in net-memcg's pressure.
> The __sk_mem_{raise,reduce}_allocated() only focus on proto/netns
> pressure rather than net-memcg's. IOW this accessor are generally
> used for deciding whether should reclaim or not.
>
> Fixes: e1aab161e013 ("socket: initial cgroup code")
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> ---
>  include/net/sock.h |  5 -----
>  net/core/sock.c    | 17 +++++++++--------
>  2 files changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 8b7ed7167243..752d51030c5a 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1404,11 +1404,6 @@ static inline int sk_under_cgroup_hierarchy(struct=
 sock *sk,
>  #endif
>  }
>
> -static inline bool sk_has_memory_pressure(const struct sock *sk)
> -{
> -       return sk->sk_prot->memory_pressure !=3D NULL;
> -}
> -
>  static inline bool sk_under_memory_pressure(const struct sock *sk)
>  {
>         if (!sk->sk_prot->memory_pressure)
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 5440e67bcfe3..8d215f821ea6 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -3017,13 +3017,14 @@ int __sk_mem_raise_allocated(struct sock *sk, int=
 size, int amt, int kind)
>                 }
>         }
>
> -       if (sk_has_memory_pressure(sk)) {
> -               u64 alloc;
> -
> -               if (!sk_under_memory_pressure(sk))
> -                       return 1;
> -               alloc =3D sk_sockets_allocated_read_positive(sk);
> -               if (sk_prot_mem_limits(sk, 2) > alloc *
> +       if (prot->memory_pressure) {

I do not understand this patch.

Changelog is evasive, I do not see what practical problem you want to solve=
.

sk_has_memory_pressure() is not about memcg, simply the fact that a
proto has a non NULL memory_pressure pointer.