Jakub Sitnicki wrote: > On Thu, Nov 03, 2022 at 02:36 PM -07, John Fastabend wrote: > > Jakub Sitnicki wrote: > >> On Tue, Nov 01, 2022 at 01:01 PM -07, John Fastabend wrote: > >> > Jakub Sitnicki wrote: > >> >> On Fri, Oct 28, 2022 at 12:16 PM -07, Cong Wang wrote: > >> >> > On Mon, Oct 24, 2022 at 03:33:13PM +0200, Jakub Sitnicki wrote: > >> >> >> On Tue, Oct 18, 2022 at 11:13 AM -07, sdf@xxxxxxxxxx wrote: > >> >> >> > On 10/17, Cong Wang wrote: > >> >> >> >> From: Cong Wang <cong.wang@xxxxxxxxxxxxx> > >> >> >> > > >> >> >> >> Technically we don't need lock the sock in the psock work, but we > >> >> >> >> need to prevent this work running in parallel with sock_map_close(). > >> >> >> > > >> >> >> >> With this, we no longer need to wait for the psock->work synchronously, > >> >> >> >> because when we reach here, either this work is still pending, or > >> >> >> >> blocking on the lock_sock(), or it is completed. We only need to cancel > >> >> >> >> the first case asynchronously, and we need to bail out the second case > >> >> >> >> quickly by checking SK_PSOCK_TX_ENABLED bit. > >> >> >> > > >> >> >> >> Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()") > >> >> >> >> Reported-by: Stanislav Fomichev <sdf@xxxxxxxxxx> > >> >> >> >> Cc: John Fastabend <john.fastabend@xxxxxxxxx> > >> >> >> >> Cc: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> > >> >> >> >> Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx> > >> >> >> > > >> >> >> > This seems to remove the splat for me: > >> >> >> > > >> >> >> > Tested-by: Stanislav Fomichev <sdf@xxxxxxxxxx> > >> >> >> > > >> >> >> > The patch looks good, but I'll leave the review to Jakub/John. > >> >> >> > >> >> >> I can't poke any holes in it either. > >> >> >> > >> >> >> However, it is harder for me to follow than the initial idea [1]. > >> >> >> So I'm wondering if there was anything wrong with it? > >> >> > > >> >> > It caused a warning in sk_stream_kill_queues() when I actually tested > >> >> > it (after posting). > >> >> > >> >> We must have seen the same warnings. They seemed unrelated so I went > >> >> digging. We have a fix for these [1]. They were present since 5.18-rc1. > >> >> > >> >> >> This seems like a step back when comes to simplifying locking in > >> >> >> sk_psock_backlog() that was done in 799aa7f98d53. > >> >> > > >> >> > Kinda, but it is still true that this sock lock is not for sk_socket > >> >> > (merely for closing this race condition). > >> >> > >> >> I really think the initial idea [2] is much nicer. I can turn it into a > >> >> patch, if you are short on time. > >> >> > >> >> With [1] and [2] applied, the dead lock and memory accounting warnings > >> >> are gone, when running `test_sockmap`. > >> >> > >> >> Thanks, > >> >> Jakub > >> >> > >> >> [1] > >> >> https://lore.kernel.org/netdev/1667000674-13237-1-git-send-email-wangyufen@xxxxxxxxxx/ > >> >> [2] https://lore.kernel.org/netdev/Y0xJUc%2FLRu8K%2FAf8@pop-os.localdomain/ > >> > > >> > Cong, what do you think? I tend to agree [2] looks nicer to me. > >> > > >> > @Jakub, > >> > > >> > Also I think we could simply drop the proposed cancel_work_sync in > >> > sock_map_close()? > >> > > >> > } > >> > @@ -1619,9 +1619,10 @@ void sock_map_close(struct sock *sk, long timeout) > >> > saved_close = psock->saved_close; > >> > sock_map_remove_links(sk, psock); > >> > rcu_read_unlock(); > >> > - sk_psock_stop(psock, true); > >> > - sk_psock_put(sk, psock); > >> > + sk_psock_stop(psock); > >> > release_sock(sk); > >> > + cancel_work_sync(&psock->work); > >> > + sk_psock_put(sk, psock); > >> > saved_close(sk, timeout); > >> > } > >> > > >> > The sk_psock_put is going to cancel the work before destroying the psock, > >> > > >> > sk_psock_put() > >> > sk_psock_drop() > >> > queue_rcu_work(system_wq, psock->rwork) > >> > > >> > and then in callback we > >> > > >> > sk_psock_destroy() > >> > cancel_work_synbc(psock->work) > >> > > >> > although it might be nice to have the work cancelled earlier rather than > >> > latter maybe. > >> > >> Good point. > >> > >> I kinda like the property that once close() returns we know there is no > >> deferred work running for the socket. > >> > >> I find the APIs where a deferred cleanup happens sometimes harder to > >> write tests for. > >> > >> But I don't really have a strong opinion here. > > > > I don't either and Cong left it so I'm good with that. > > > > Reviewing backlog logic though I think there is another bug there, but > > I haven't been able to trigger it in any of our tests. > > > > The sk_psock_backlog() logic is, > > > > sk_psock_backlog(struct work_struct *work) > > mutex_lock() > > while (skb = ...) > > ... > > do { > > ret = sk_psock_handle_skb() > > if (ret <= 0) { > > if (ret == -EAGAIN) { > > sk_psock_skb_state() > > goto end; > > } > > ... > > } while (len); > > ... > > end: > > mutex_unlock() > > > > what I'm not seeing is if we get an EAGAIN through sk_psock_handle_skb > > how do we schedule the backlog again. For egress we would set the > > SOCK_NOSPACE bit and then get a write space available callback which > > would do the schedule(). The ingress side could fail with EAGAIN > > through the alloc_sk_msg(GFP_ATOMIC) call. This is just a kzalloc, > > > > sk_psock_handle_skb() > > sk_psock_skb_ingress() > > sk_psock_skb_ingress_self() > > msg = alloc_sk_msg() > > kzalloc() <- this can return NULL > > if (!msg) > > return -EAGAIN <- could we stall now > > > > > > I think we could stall here if there was nothing else to kick it. I > > was thinking about this maybe, > > > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c > > index 1efdc47a999b..b96e95625027 100644 > > --- a/net/core/skmsg.c > > +++ b/net/core/skmsg.c > > @@ -624,13 +624,20 @@ static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb, > > static void sk_psock_skb_state(struct sk_psock *psock, > > struct sk_psock_work_state *state, > > struct sk_buff *skb, > > - int len, int off) > > + int len, int off, bool ingress) > > { > > spin_lock_bh(&psock->ingress_lock); > > if (sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED)) { > > state->skb = skb; > > state->len = len; > > state->off = off; > > + /* For ingress we may not have a wakeup callback to trigger > > + * the reschedule on so need to reschedule retry. For egress > > + * we will get TCP stack callback when its a good time to > > + * retry. > > + */ > > + if (ingress) > > + schedule_work(&psock->work); > > } else { > > sock_drop(psock->sk, skb); > > } > > @@ -678,7 +685,7 @@ static void sk_psock_backlog(struct work_struct *work) > > if (ret <= 0) { > > if (ret == -EAGAIN) { > > sk_psock_skb_state(psock, state, skb, > > - len, off); > > + len, off, ingress); > > goto end; > > } > > /* Hard errors break pipe and stop xmit. */ > > > > > > Its tempting to try and use the memory pressure callbacks but those are > > built for the skb cache so I think overloading them is not so nice. The > > drawback to above is its possible no memory is available even when we > > get back to the backlog. We could use a delayed reschedule but its not > > clear what delay makes sense here. Maybe some backoff... > > > > Any thoughts? > > I don't have any thoughts on the fix yet, but I have a repro. I'm testing it with a delayed workqueue now and a backoff just so we don't bang on this repeatedly when OOM condition is met. Then all the other schedule_work() calls become the delayed variant but I think this is OK. Better ideas welcome but running the above through our CI today. > > We can use fault injection [1]. For some reason it's been disabled on > x86-64 since 2007 (stack walking didn't work back then?), so we need to > patch the kernel slightly. Could add the function to ALLOW_OVERRIDE as well. But not sure we want to force it to be _not_ inlined in general case. > > Also, to better target the failure, just for this case, I've de-inlined > alloc_sk_msg(). But in general testing we can just inject any alloc > under sk_psock_backlog(). > > Incantation looks like so: > > #!/usr/bin/env bash > > readonly TARGET_FUNC=alloc_sk_msg > readonly ADDR=($(grep -A1 ${TARGET_FUNC} /proc/kallsyms | awk '{print "0x" $1}')) > > exec bash \ > ../../fault-injection/failcmd.sh \ > --require-start=${ADDR[0]} --require-end=${ADDR[1]} \ > --stacktrace-depth=32 \ > --probability=50 --times=100 \ > --ignore-gfp-wait=N --task-filter=N \ > -- \ > ./test_sockmap > > We won't get a message in dmesg (even with --verbosity=1 set) because > we're allocating with __GFP_NOWARN, and fault injection interface > doesn't provide a way to override that. But we can obseve the 'times' > count go down after ./test_sockmap blocks (also confirmed with a printk > added on -EAGAIN error path). We can probably do it through BPF prog with ALLOW_OVERRIDE on one of those functions in that call path then we can write a selftest for it. > > This is what I observe: Very cool. > > bash-5.1# ./repro.sh > # 1/ 6 sockmap::txmsg test passthrough:OK > # 2/ 6 sockmap::txmsg test redirect:OK > # 3/ 1 sockmap::txmsg test redirect wait send mem:OK > # 4/ 6 sockmap::txmsg test drop:OK > # 5/ 6 sockmap::txmsg test ingress redirect:OK <-- blocked here > ^Z > [1]+ Stopped ./repro.sh > bash-5.1# cat /sys/kernel/debug/failslab/times > 99 > bash-5.1# > [...]