Re: [PATCH net-next 3/5] net: batch zerocopy_fill_skb_from_iter accounting

Pavel Begunkov
2024-07-01T11:11:04Z

On 6/28/24 18:06, Willem de Bruijn wrote:
> Pavel Begunkov wrote:
>> Instead of accounting every page range against the socket separately, do
>> it in batch based on the change in skb->truesize. It's also moved into
>> __zerocopy_sg_from_iter(), so that zerocopy_fill_skb_from_iter() is
>> simpler and responsible for setting frags but not the accounting.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>
> Reviewed-by: Willem de Bruijn <willemb@google.com>

Thanks for reviews!

>> ---
>> net/core/datagram.c | 31 ++++++++++++++++++-------------
>> 1 file changed, 18 insertions(+), 13 deletions(-)
>>
>> diff --git a/net/core/datagram.c b/net/core/datagram.c
>> index 7f7d5da2e406..2b24d69b1e94 100644
>> --- a/net/core/datagram.c
>> +++ b/net/core/datagram.c
>> @@ -610,7 +610,7 @@ int skb_copy_datagram_from_iter(struct sk_buff *skb, int offset,
>> }
>> EXPORT_SYMBOL(skb_copy_datagram_from_iter);
>>
>> -static int zerocopy_fill_skb_from_iter(struct sock *sk, struct sk_buff *skb,
>> +static int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
>> struct iov_iter *from, size_t length)
>> {
>> int frag = skb_shinfo(skb)->nr_frags;
>> @@ -621,7 +621,6 @@ static int zerocopy_fill_skb_from_iter(struct sock *sk, struct sk_buff *skb,
>> int refs, order, n = 0;
>> size_t start;
>> ssize_t copied;
>> - unsigned long truesize;
>>
>> if (frag == MAX_SKB_FRAGS)
>> return -EMSGSIZE;
>
> Does the existing code then incorrectly not unwind sk_wmem_queued_add
> and sk_mem_charge if returning with error from the second or later
> loop..

As long as ->truesize matches what's accounted to the socket,
kfree_skb() -> sock_wfree()/->destructor() should take care of it.
With sk_mem_charge() I assume __zerocopy_sg_from_iter -> ___pskb_trim()
should do it, need to look it up, but if not, it sounds like a temporary
over estimation until the skb is put down. I don't see anything
concerning. Is that the scenario you're worried about?

--
Pavel Begunkov