netdev - Re: [PATCH v1 bpf-next 03/10] bpf: tcp: Get rid of st_bucket

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Message-ID: <20250522204443.78455-1-kuniyu@amazon.com>
Date: Thu, 22 May 2025 13:42:02 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <jordan@...fe.io>
CC: <alexei.starovoitov@...il.com>, <bpf@...r.kernel.org>,
	<daniel@...earbox.net>, <kuniyu@...zon.com>, <martin.lau@...ux.dev>,
	<netdev@...r.kernel.org>, <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH v1 bpf-next 03/10] bpf: tcp: Get rid of st_bucket_done

From: Jordan Rife <jordan@...fe.io>
Date: Thu, 22 May 2025 11:16:13 -0700
> > > >  static void bpf_iter_tcp_put_batch(struct bpf_tcp_iter_state *iter)
> > > >  {
> > > > -	while (iter->cur_sk < iter->end_sk)
> > > > -		sock_gen_put(iter->batch[iter->cur_sk++]);
> > > > +	unsigned int cur_sk = iter->cur_sk;
> > > > +
> > > > +	while (cur_sk < iter->end_sk)
> > > > +		sock_gen_put(iter->batch[cur_sk++]);
> > > 
> > > Why is this chunk included in this patch ?
> > 
> > This should be in patch 5 to keep cur_sk for find_cookie
> 
> Without this, iter->cur_sk is mutated when iteration stops, and we lose
> our place. When iteration resumes and we call bpf_iter_tcp_batch the
> iter->cur_sk == iter->end_sk condition will always be true, so we will
> skip to the next bucket without seeking to the offset.
> 
> Before, we relied on st_bucket_done to tell us if we had remaining items
> in the current bucket to process but now need to preserve iter->cur_sk
> through iterations to make the behavior equivalent to what we had before.

Thanks for explanation, I was confused by calling tcp_seek_last_pos()
multiple times, and I think we need to preserve/restore st->offset too
in patch 2 and need this change.

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ac00015d5e7a..0816f20bfdff 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2791,6 +2791,7 @@ static void *tcp_seek_last_pos(struct seq_file *seq)
 			break;
 		st->bucket = 0;
 		st->state = TCP_SEQ_STATE_ESTABLISHED;
+		offset = 0;
 		fallthrough;
 	case TCP_SEQ_STATE_ESTABLISHED:
 		if (st->bucket > hinfo->ehash_mask)


Let's say we are resuming at an offset (10) in the last lhash bucket
but a few sockets (3) disappeared, then we go to the ehash part with
a non-zero offset (3), which will overwrite st->offset (3).

If the ehash does not fit into the batch size, we need to allocate
a new batch and retry, but the offset (3) is different from the
first try (10).