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

[Muchun Song]

On Tue, Oct 13, 2020 at 5:47 AM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>
> On Sun, Oct 11, 2020 at 9:22 PM 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
>
> Interesting, we already have a few counters related to socket buffers,
> are you saying these are not accounted in /proc/meminfo either?

Yeah, these are not accounted for in /proc/meminfo.

> 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;
>

Yeah, it is already accounted for. But it 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 page frag memory
allocation path.

>
> >
> > >
> > > >  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.

Yeah, it looks a little strange. I just want to account for page frag
allocations. So I have to use PG_private to distinguish the page
from page frag or others in the __skb_frag_unref(). If the page is
allocated from skb_page_frag_refill, we should decrease the
statistics.

Thanks.

>
> Thanks.



-- 
Yours,
Muchun