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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Pavel Begunkov wrote:
> 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?

Oh indeed. Thanks. I don't see ___pskb_trim adjusting except for the
cases where it calls skb_condese, but neither does it adjust truesize.
So indeed a temporary over estimation until e.g., tcp_wmem_free_skb.
Sounds fine.