On 2/12/25 19:18, Mina Almasry wrote:
On Wed, Feb 12, 2025 at 7:52 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
On 2/10/25 21:09, Mina Almasry wrote:
On Wed, Feb 5, 2025 at 4:20 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
On 2/3/25 22:39, Mina Almasry wrote:
...
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index bb2b751d274a..3ff8f568c382 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1711,9 +1711,12 @@ struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
...
int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
struct iov_iter *from, size_t length);
@@ -1721,12 +1724,14 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,
struct msghdr *msg, int len)
{
- return __zerocopy_sg_from_iter(msg, skb->sk, skb, &msg->msg_iter, len);
+ return __zerocopy_sg_from_iter(msg, skb->sk, skb, &msg->msg_iter, len,
+ NULL);
Instead of propagating it all the way down and carving a new path, why
not reuse the existing infra? You already hook into where ubuf is
allocated, you can stash the binding in there. And
It looks like it's not possible to increase the side of ubuf_info at
all, otherwise the BUILD_BUG_ON in msg_zerocopy_alloc() fires.
It's asserting that sizeof(ubuf_info_msgzc) <= sizeof(skb->cb), and
I'm guessing increasing skb->cb size is not really the way to go.
What I may be able to do here is stash the binding somewhere in
ubuf_info_msgzc via union with fields we don't need for devmem, and/or
It doesn't need to account the memory against the user, and you
actually don't want that because dmabuf should take care of that.
So, it should be fine to reuse ->mmp.
It's also not a real sk_buff, so maybe maintainers wouldn't mind
reusing some more space out of it, if that would even be needed.
netmem skb are real sk_buff, with the modification that frags are not
We were discussing ubuf_info allocation, take a look at
msg_zerocopy_alloc(), it has nothing to do with netmems and all that.
readable, only in the case that the netmem is unreadable. I would not
approve of considering netmem/devmem skbs "not real skbs", and start
messing with the semantics of skb fields for devmem skbs, and having
to start adding skb_is_devmem() checks through all code in the skb
handlers that touch the fields being overwritten in the devmem case.
No, I don't think we can re-use random fields in the skb for devmem.
stashing the binding in ubuf_info_ops (very hacky). Neither approach
seems ideal, but the former may work and may be cleaner.
I'll take a deeper look here. I had looked before and concluded that
we're piggybacking devmem TX on MSG_ZEROCOPY path, because we need
almost all of the functionality there (no copying, send complete
notifications, etc), with one minor change in the skb filling. I had
concluded that if MSG_ZEROCOPY was never updated to use the existing
infra, then it's appropriate for devmem TX piggybacking on top of it
MSG_ZEROCOPY does use the common infra, i.e. passing ubuf_info,
but doesn't need ->sg_from_iter as zerocopy_fill_skb_from_iter()
and it's what was there first.
But MSG_ZEROCOPY doesn't set msg->msg_ubuf. And not setting
msg->msg_ubuf fails to trigger msg->sg_from_iter altogether.
And also currently sg_from_iter isn't set up to take in a ubuf_info.
We'd need that if we stash the binding in the ubuf_info.
https://github.com/isilence/linux.git sg-iter-ops
I have old patches for all of that, they even rebased cleanly. That
should do it for you, and I need to send then regardless of devmem.
All in all I think I wanna prototype an msg->sg_from_iter approach and
make a judgement call on whether it's cleaner than just passing the
binding through a couple of helpers just as I'm doing here. My feeling
is that the implementation in this patch may be cleaner than
refactoring the entire msg_ubuf/sg_from_iter flows so we can sort of
use it for MSG_ZEROCOPY with devmem when it currently doesn't use it.
to follow that. I would not want to get into a refactor of
MSG_ZEROCOPY for no real reason.
But I'll take a deeper look here and see if I can make something
slightly cleaner work.
--
Pavel Begunkov
