Re: [External] Re: [PATCH] mm: proc: add Sock to /proc/meminfo

[Eric Dumazet <edumazet@google.com>]

On Mon, Oct 12, 2020 at 6:22 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Mon, Oct 12, 2020 at 2:39 AM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >
> > On Sat, Oct 10, 2020 at 3:39 AM Muchun Song <songmuchun@bytedance.com> wrote:
> > >
> > > The amount of memory allocated to sockets buffer can become significant.
> > > However, we do not display the amount of memory consumed by sockets
> > > buffer. In this case, knowing where the memory is consumed by the kernel
> >
> > We do it via `ss -m`. Is it not sufficient? And if not, why not adding it there
> > rather than /proc/meminfo?
>
> If the system has little free memory, we can know where the memory is via
> /proc/meminfo. If a lot of memory is consumed by socket buffer, we cannot
> know it when the Sock is not shown in the /proc/meminfo. If the unaware user
> can't think of the socket buffer, naturally they will not `ss -m`. The
> end result
> is that we still don’t know where the memory is consumed. And we add the
> Sock to the /proc/meminfo just like the memcg does('sock' item in the cgroup
> v2 memory.stat). So I think that adding to /proc/meminfo is sufficient.
>
> >
> > >  static inline void __skb_frag_unref(skb_frag_t *frag)
> > >  {
> > > -       put_page(skb_frag_page(frag));
> > > +       struct page *page = skb_frag_page(frag);
> > > +
> > > +       if (put_page_testzero(page)) {
> > > +               dec_sock_node_page_state(page);
> > > +               __put_page(page);
> > > +       }
> > >  }
> >
> > You mix socket page frag with skb frag at least, not sure this is exactly
> > what you want, because clearly skb page frags are frequently used
> > by network drivers rather than sockets.
> >
> > Also, which one matches this dec_sock_node_page_state()? Clearly
> > not skb_fill_page_desc() or __skb_frag_ref().
>
> Yeah, we call inc_sock_node_page_state() in the skb_page_frag_refill().
> So if someone gets the page returned by skb_page_frag_refill(), it must
> put the page via __skb_frag_unref()/skb_frag_unref(). We use PG_private
> to indicate that we need to dec the node page state when the refcount of
> page reaches zero.
>

Pages can be transferred from pipe to socket, socket to pipe (splice()
and zerocopy friends...)

 If you want to track TCP memory allocations, you always can look at
/proc/net/sockstat,
without adding yet another expensive memory accounting.