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SubjectRe: [PATCH 9/9] net: vm deadlock avoidance core
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On Tue, 2007-01-16 at 16:25 +0300, Evgeniy Polyakov wrote:
> On Tue, Jan 16, 2007 at 10:46:06AM +0100, Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:

> > @@ -1767,10 +1767,23 @@ int netif_receive_skb(struct sk_buff *sk
> > struct net_device *orig_dev;
> > int ret = NET_RX_DROP;
> > __be16 type;
> > + unsigned long pflags = current->flags;
> > +
> > + /* Emergency skb are special, they should
> > + * - be delivered to SOCK_VMIO sockets only
> > + * - stay away from userspace
> > + * - have bounded memory usage
> > + *
> > + * Use PF_MEMALLOC as a poor mans memory pool - the grouping kind.
> > + * This saves us from propagating the allocation context down to all
> > + * allocation sites.
> > + */
> > + 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?

I thought this was softirq context and thus this would either run in a
borrowed context or in ksoftirqd. See patch 3/9.

> > @@ -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?

> > #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?

> > +}
> > +
> > /*
> > * 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.

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