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

[Mike Rapoport]

On Mon, Oct 12, 2020 at 05:53:01PM +0800, Muchun Song wrote:
> On Mon, Oct 12, 2020 at 5:24 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> >
> > On 10/12/20 10:39 AM, Muchun Song wrote:
> > > On Mon, Oct 12, 2020 at 3:42 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > >>
> > >> On Mon, Oct 12, 2020 at 6:22 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > >>>
> > >>> On Mon, Oct 12, 2020 at 2:39 AM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> > >>>>
> > >>>> On Sat, Oct 10, 2020 at 3:39 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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.
> > >
> > > The 'mem' item in the /proc/net/sockstat does not represent real
> > > memory usage. This is just the total amount of charged memory.
> > >
> > > For example, if a task sends a 10-byte message, it only charges one
> > > page to memcg. But the system may allocate 8 pages. Therefore, it
> > > does not truly reflect the memory allocated by the above memory
> > > allocation path. We can see the difference via the following message.
> > >
> > > cat /proc/net/sockstat
> > >   sockets: used 698
> > >   TCP: inuse 70 orphan 0 tw 617 alloc 134 mem 13
> > >   UDP: inuse 90 mem 4
> > >   UDPLITE: inuse 0
> > >   RAW: inuse 1
> > >   FRAG: inuse 0 memory 0
> > >
> > > cat /proc/meminfo | grep Sock
> > >   Sock:              13664 kB
> > >
> > > The /proc/net/sockstat only shows us that there are 17*4 kB TCP
> > > memory allocations. But apply this patch, we can see that we truly
> > > allocate 13664 kB(May be greater than this value because of per-cpu
> > > stat cache). Of course the load of the example here is not high. In
> > > some high load cases, I believe the difference here will be even
> > > greater.
> > >
> >
> > This is great, but you have not addressed my feedback.
> >
> > TCP memory allocations are bounded by /proc/sys/net/ipv4/tcp_mem
> >
> > Fact that the memory is forward allocated or not is a detail.
> >
> > If you think we must pre-allocate memory, instead of forward allocations,
> > your patch does not address this. Adding one line per consumer in /proc/meminfo looks
> > wrong to me.
> 
> I think that the consumer which consumes a lot of memory should be added
> to the /proc/meminfo. This can help us know the user of large memory.
> 
> >
> > If you do not want 9.37 % of physical memory being possibly used by TCP,
> > just change /proc/sys/net/ipv4/tcp_mem accordingly ?
> 
> We are not complaining about TCP using too much memory, but how do
> we know that TCP uses a lot of memory. When I firstly face this problem,
> I do not know who uses the 25GB memory and it is not shown in the /proc/meminfo.
> If we can know the amount memory of the socket buffer via /proc/meminfo, we
> may not need to spend a lot of time troubleshooting this problem. Not everyone
> knows that a lot of memory may be used here. But I believe many people
> should know /proc/meminfo to confirm memory users.

If I undestand correctly, the problem you are trying to solve is to
simplify troubleshooting of memory usage for people who may not be aware
that networking stack can be a large memory consumer.

For that a paragraph in 'man 5 proc' maybe a good start: