Willem de Bruijn <willemdebruijn.ker...@gmail.com> wrote: >On Thu, Aug 13, 2020 at 1:59 PM Miaohe Lin <linmia...@huawei.com> wrote: >> >> The var extra_uref is introduced to pass the initial reference taken >> in sock_zerocopy_alloc to the first generated skb. But now we may fail >> to pass the initial reference with newly allocated UDP or RAW uarg >> when the skb is zcopied. > >extra_uref is true if there is no previous skb to append to or there is a >previous skb, but that does not have zerocopy data associated yet (because the >previous call(s) did not set MSG_ZEROCOPY). > >In other words, when first (allocating and) associating a zerocopy struct with >the skb.
Many thanks for your explaination. The var extra_uref plays the role as you say. I just borrowed the description of var extra_uref from previous commit log here. > >> - extra_uref = !skb_zcopy(skb); /* only ref on new uarg */ >> + /* Only ref on newly allocated uarg. */ >> + if (!skb_zcopy(skb) || (sk->sk_type != SOCK_STREAM && >> skb_zcopy(skb) != uarg)) >> + extra_uref = true; > >SOCK_STREAM does not use __ip_append_data. > >This leaves as new branch skb_zcopy(skb) && skb_zcopy(skb) != uarg. > >This function can only acquire a uarg through sock_zerocopy_realloc, which on >skb_zcopy(skb) only returns the existing uarg or NULL (for not SOCK_STREAM). > >So I don't see when that condition can happen. > On skb_zcopy(skb), we returns the existing uarg iff (uarg->id + uarg->len == atomic_read(&sk->sk_zckey)) in sock_zerocopy_realloc. So we may get a newly allocated uarg via sock_zerocopy_alloc(). Though we may not trigger this codepath now, it's still a potential problem that we may missed the right trace to uarg. Or am I still miss anything? Thanks.
