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Date: Tue, 16 Jan 2007 18:33:15 +0300
From: Evgeniy Polyakov <johnpol@....mipt.ru>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
linux-mm@...ck.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH 9/9] net: vm deadlock avoidance core
On Tue, Jan 16, 2007 at 02:47:54PM +0100, Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> > > + if (unlikely(skb->emergency))
> > > + current->flags |= PF_MEMALLOC;
> >
> > Access to 'current' in netif_receive_skb()???
> > Why do you want to work with, for example keventd?
>
> Can this run in keventd?
Initial netchannel implementation by Kelly Daly (IBM) worked in keventd
(or dedicated kernel thread, I do not recall).
> I thought this was softirq context and thus this would either run in a
> borrowed context or in ksoftirqd. See patch 3/9.
And how are you going to access 'current' in softirq?
netif_receive_skb() can also be called from a lot of other places
including keventd and/or different context - it is permitted to call it
everywhere to process packet.
I meant that you break the rule accessing 'current' in that context.
> > > @@ -1798,6 +1811,8 @@ int netif_receive_skb(struct sk_buff *sk
> > > goto ncls;
> > > }
> > > #endif
> > > + if (unlikely(skb->emergency))
> > > + goto skip_taps;
> > >
> > > list_for_each_entry_rcu(ptype, &ptype_all, list) {
> > > if (!ptype->dev || ptype->dev == skb->dev) {
> > > @@ -1807,6 +1822,7 @@ int netif_receive_skb(struct sk_buff *sk
> > > }
> > > }
> > >
> > > +skip_taps:
> >
> > It is still a 'tap'.
>
> Not sure what you are saying, I thought this should stop delivery of
> skbs to taps?
Ingres filter can do whatever it wants with skb at that point, likely
you want to skip that hunk too.
> > > #ifdef CONFIG_NET_CLS_ACT
> > > if (pt_prev) {
> > > ret = deliver_skb(skb, pt_prev, orig_dev);
> > > @@ -1819,15 +1835,26 @@ int netif_receive_skb(struct sk_buff *sk
> > >
> > > if (ret == TC_ACT_SHOT || (ret == TC_ACT_STOLEN)) {
> > > kfree_skb(skb);
> > > - goto out;
> > > + goto unlock;
> > > }
> > >
> > > skb->tc_verd = 0;
> > > ncls:
> > > #endif
> > >
> > > + if (unlikely(skb->emergency))
> > > + switch(skb->protocol) {
> > > + case __constant_htons(ETH_P_ARP):
> > > + case __constant_htons(ETH_P_IP):
> > > + case __constant_htons(ETH_P_IPV6):
> > > + break;
> >
> > Poor vlans and appletalk.
>
> Yeah and all those other too, maybe some day.
>
> > > Index: linux-2.6-git/net/ipv4/tcp_ipv4.c
> > > ===================================================================
> > > --- linux-2.6-git.orig/net/ipv4/tcp_ipv4.c 2007-01-12 12:20:07.000000000 +0100
> > > +++ linux-2.6-git/net/ipv4/tcp_ipv4.c 2007-01-12 12:21:14.000000000 +0100
> > > @@ -1604,6 +1604,22 @@ csum_err:
> > > goto discard;
> > > }
> > >
> > > +static int tcp_v4_backlog_rcv(struct sock *sk, struct sk_buff *skb)
> > > +{
> > > + int ret;
> > > + unsigned long pflags = current->flags;
> > > + if (unlikely(skb->emergency)) {
> > > + BUG_ON(!sk_has_vmio(sk)); /* we dropped those before queueing */
> > > + if (!(pflags & PF_MEMALLOC))
> > > + current->flags |= PF_MEMALLOC;
> > > + }
> > > +
> > > + ret = tcp_v4_do_rcv(sk, skb);
> > > +
> > > + current->flags = pflags;
> > > + return ret;
> >
> > Why don't you want to just setup PF_MEMALLOC for the socket and all
> > related processes?
>
> I'm not understanding what you're saying here.
>
> I want grant the processing of skb->emergency packets access to the
> memory reserves.
>
> How would I set PF_MEMALLOC on a socket, its a process flag? And which
> related processes?
You use special flag for sockets to mark them as capable of
'reserve-eating', too many flags are a bit confusing.
I meant that you can just mark process which created such socket as
PF_MEMALLOC, and clone that flag on forks and other relatest calls without
all that checks for 'current' in different places.
> > > +}
> > > +
> > > /*
> > > * From tcp_input.c
> > > */
> > > @@ -1654,6 +1670,15 @@ int tcp_v4_rcv(struct sk_buff *skb)
> > > if (!sk)
> > > goto no_tcp_socket;
> > >
> > > + if (unlikely(skb->emergency)) {
> > > + if (!sk_has_vmio(sk))
> > > + goto discard_and_relse;
> > > + /*
> > > + decrease window size..
> > > + tcp_enter_quickack_mode(sk);
> > > + */
> >
> > How does this decrease window size?
> > Maybe ack scheduling would be better handled by inet_csk_schedule_ack()
> > or just directly send an ack, which in turn requires allocation, which
> > can be bound to this received frame processing...
>
> It doesn't, I thought that it might be a good idea doing that, but never
> got around to actually figuring out how to do it.
tcp_send_ack()?
--
Evgeniy Polyakov
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