On 12/21/23 12:27 PM, Daniel Borkmann wrote:
On 12/21/23 3:58 PM, Martin KaFai Lau wrote:
On 12/21/23 5:21 AM, Daniel Borkmann wrote:
On 12/21/23 5:45 AM, Martin KaFai Lau wrote:
On 12/20/23 11:10 AM, Martin KaFai Lau wrote:
Good catch. It will unnecessary skip in the following batch/bucket if there
is changes in the current batch/bucket.
From looking at the loop again, I think it is better not to change the
iter->offset during the for loop. Only update iter->offset after the for
loop has concluded.
The non-zero iter->offset is only useful for the first bucket, so does a
test on the first bucket (state->bucket == bucket) before skipping sockets.
Something like this:
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 89e5a806b82e..a993f364d6ae 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -3139,6 +3139,7 @@ static struct sock *bpf_iter_udp_batch(struct
seq_file *seq)
struct net *net = seq_file_net(seq);
struct udp_table *udptable;
unsigned int batch_sks = 0;
+ int bucket, bucket_offset;
bool resized = false;
struct sock *sk;
@@ -3162,14 +3163,14 @@ static struct sock *bpf_iter_udp_batch(struct
seq_file *seq)
iter->end_sk = 0;
iter->st_bucket_done = false;
batch_sks = 0;
+ bucket = state->bucket;
+ bucket_offset = 0;
for (; state->bucket <= udptable->mask; state->bucket++) {
struct udp_hslot *hslot2 = &udptable->hash2[state->bucket];
- if (hlist_empty(&hslot2->head)) {
- iter->offset = 0;
+ if (hlist_empty(&hslot2->head))
continue;
- }
spin_lock_bh(&hslot2->lock);
udp_portaddr_for_each_entry(sk, &hslot2->head) {
@@ -3177,8 +3178,9 @@ static struct sock *bpf_iter_udp_batch(struct
seq_file *seq)
/* Resume from the last iterated socket at the
* offset in the bucket before iterator was stopped.
*/
- if (iter->offset) {
- --iter->offset;
+ if (state->bucket == bucket &&
+ bucket_offset < iter->offset) {
+ ++bucket_offset;
continue;
}
if (iter->end_sk < iter->max_sk) {
@@ -3192,10 +3194,10 @@ static struct sock *bpf_iter_udp_batch(struct
seq_file *seq)
if (iter->end_sk)
break;
+ }
- /* Reset the current bucket's offset before moving to the next
bucket. */
+ if (state->bucket != bucket)
iter->offset = 0;
- }
/* All done: no batch made. */
if (!iter->end_sk)
I think I found another bug in the current bpf_iter_udp_batch(). The
"state->bucket--;" at the end of the batch() function is wrong also. It does
not need to go back to the previous bucket. After realloc with a larger
batch array, it should retry on the "state->bucket" as is. I tried to force
the bind() to use bucket 0 and bind a larger so_reuseport set (24 sockets).
WARN_ON(state->bucket < 0) triggered.
Going back to this bug (backward progress on --iter->offset), I think it is
a bit cleaner to always reset iter->offset to 0 and advance iter->offset to
the resume_offset only when needed. Something like this:
Hm, my assumption was.. why not do something like the below, and fully start
over?
I'm mostly puzzled about the side-effects here, in particular, if for the
rerun the sockets
in the bucket could already have changed.. maybe I'm still missing something
- what do
we need to deal with exactly worst case when we need to go and retry
everything, and what
guarantees do we have?
(only compile tested)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 89e5a806b82e..ca62a4bb7bec 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -3138,7 +3138,8 @@ static struct sock *bpf_iter_udp_batch(struct seq_file
*seq)
struct udp_iter_state *state = &iter->state;
struct net *net = seq_file_net(seq);
struct udp_table *udptable;
- unsigned int batch_sks = 0;
+ int orig_bucket, orig_offset;
+ unsigned int i, batch_sks = 0;
bool resized = false;
struct sock *sk;
@@ -3149,7 +3150,8 @@ static struct sock *bpf_iter_udp_batch(struct seq_file
*seq)
}
udptable = udp_get_table_seq(seq, net);
-
+ orig_bucket = state->bucket;
+ orig_offset = iter->offset;
again:
/* New batch for the next bucket.
* Iterate over the hash table to find a bucket with sockets matching
@@ -3211,9 +3213,15 @@ static struct sock *bpf_iter_udp_batch(struct seq_file
*seq)
if (!resized && !bpf_iter_udp_realloc_batch(iter, batch_sks * 3 / 2)) {
resized = true;
/* After allocating a larger batch, retry one more time to grab
- * the whole bucket.
+ * the whole bucket. Drop the current refs since for the next
+ * attempt the composition could have changed, thus start over.
*/
- state->bucket--;
+ for (i = 0; i < iter->end_sk; i++) {
+ sock_put(iter->batch[i]);
+ iter->batch[i] = NULL;
+ }
+ state->bucket = orig_bucket;
+ iter->offset = orig_offset;
It does not need to start over from the orig_bucket. Once it advanced to the
next bucket (state->bucket++), the orig_bucket is done. Otherwise, it may need
to make backward progress here on the state->bucket. The batch size too small
happens on the current state->bucket, so it should retry with the same
state->bucket after realloc_batch(). If the state->bucket happens to be the
orig_bucket (mean it has not advanced), it will skip the same orig_offset.
If the orig_bucket had changed (e.g. having more sockets than the last time it
was batched) after state->bucket++, it is arguably fine because it was added
after the orig_bucket was completely captured in a batch before. The same goes
for (orig_bucket-1) that could have changed during the whole udp_table iteration.
Fair, I was thinking in relation to the above to avoid such inconsistency, hence
the drop of the refs.
Ah, for the ref drop, the bpf_iter_udp_realloc_batch() earlier did the sock_put
on the iter->batch[]. It is done in the bpf_iter_udp_put_batch(). When it
retries with a larger iter->batch[] array, it does retry from the very first
udp_sk of the state->bucket again. Thus, the intention is to do the best effort
to capture the whole bucket under the "spin_lock_bh(&hslot2->lock)".
I think it's probably fine to argue either way on how semantics
should be as long as the code doesn't get overly complex for covering this corner
case.
Thanks,
Daniel
