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

Cong Wang
2020-10-12T21:47:05Z

On Sun, Oct 11, 2020 at 9:22 PM 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

Interesting, we already have a few counters related to socket buffers,
are you saying these are not accounted in /proc/meminfo either?
If yes, why are page frags so special here? If not, they are more
important than page frags, so you probably want to deal with them
first.

> 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.

It looks like actually the socket page frag is already accounted,
for example, the tcp_sendmsg_locked():

copy = min_t(int, copy, pfrag->size - pfrag->offset);

if (!sk_wmem_schedule(sk, copy))
goto wait_for_memory;

>
> >
> > > 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().

How is skb_page_frag_refill() possibly paired with __skb_frag_unref()?

> 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.

skb_page_frag_refill() is called on frags not within an skb, for instance,
sk_page_frag_refill() uses it for a per-socket or per-process page frag.
But, __skb_frag_unref() is specifically used for skb frags, which are
supposed to be filled by skb_fill_page_desc() (page is allocated by driver).

They are different things you are mixing them up, which looks clearly
wrong or at least misleading.

Thanks.