Re: [PATCH net-next 0/4] shrink struct ubuf_info

[Pavel Begunkov]

On 9/27/22 18:56, Paolo Abeni wrote:
> On Tue, 2022-09-27 at 18:16 +0100, Pavel Begunkov wrote:
> > On 9/27/22 15:28, Pavel Begunkov wrote:
> > > Hello Paolo,
> > >
> > > On 9/27/22 14:49, Paolo Abeni wrote:
> > > > Hello,
> > > >
> > > > On Fri, 2022-09-23 at 17:39 +0100, Pavel Begunkov wrote:
> > > > > struct ubuf_info is large but not all fields are needed for all
> > > > > cases. We have limited space in io_uring for it and large ubuf_info
> > > > > prevents some struct embedding, even though we use only a subset
> > > > > of the fields. It's also not very clean trying to use this typeless
> > > > > extra space.
> > > > >
> > > > > Shrink struct ubuf_info to only necessary fields used in generic paths,
> > > > > namely ->callback, ->refcnt and ->flags, which take only 16 bytes. And
> > > > > make MSG_ZEROCOPY and some other users to embed it into a larger struct
> > > > > ubuf_info_msgzc mimicking the former ubuf_info.
> > > > >
> > > > > Note, xen/vhost may also have some cleaning on top by creating
> > > > > new structs containing ubuf_info but with proper types.
> > > >
> > > > That sounds a bit scaring to me. If I read correctly, every uarg user
> > > > should check 'uarg->callback == msg_zerocopy_callback' before accessing
> > > > any 'extend' fields.
> > >
> > > Providers of ubuf_info access those fields via callbacks and so already
> > > know the actual structure used. The net core, on the opposite, should
> > > keep it encapsulated and not touch them at all.
> > >
> > > The series lists all places where we use extended fields just on the
> > > merit of stripping the structure of those fields and successfully
> > > building it. The only user in net/ipv{4,6}/* is MSG_ZEROCOPY, which
> > > again uses callbacks.
> > >
> > > Sounds like the right direction for me. There is a couple of
> > > places where it might get type safer, i.e. adding types instead
> > > of void* in for struct tun_msg_ctl and getting rid of one macro
> > > hiding types in xen. But seems more like TODO for later.
> > >
> > > > AFAICS the current code sometimes don't do the
> > > > explicit test because the condition is somewhat implied, which in turn
> > > > is quite hard to track.
> > > >
> > > > clearing uarg->zerocopy for the 'wrong' uarg was armless and undetected
> > > > before this series, and after will trigger an oops..
> > >
> > > And now we don't have this field at all to access, considering that
> > > nobody blindly casts it.
> > >
> > > > There is some noise due to uarg -> uarg_zc renaming which make the
> > > > series harder to review. Have you considered instead keeping the old
> > > > name and introducing a smaller 'struct ubuf_info_common'? the overall
> > > > code should be mostly the same, but it will avoid the above mentioned
> > > > noise.
> > >
> > > I don't think there will be less noise this way, but let me try
> > > and see if I can get rid of some churn.
> >
> > It doesn't look any better for me
> >
> > TL;DR; This series converts only 3 users: tap, xen and MSG_ZEROCOPY
> > and doesn't touch core code. If we do ubuf_info_common though I'd need
> > to convert lots of places in skbuff.c and multiple places across
> > tcp/udp, which is much worse.
>
> Uhmm... I underlook the fact we must preserve the current accessors for
> the common fields.
>
> I guess something like the following could do (completely untested,
> hopefully should illustrate the idea):
>
> struct ubuf_info {
> 	struct_group_tagged(ubuf_info_common, common,
> 		void (*callback)(struct sk_buff *, struct ubuf_info *,
>                           bool zerocopy_success);
> 		refcount_t refcnt;
> 	        u8 flags;
> 	);
>
> 	union {
>                  struct {
>                          unsigned long desc;
>                          void *ctx;
>                  };
>                  struct {
>                          u32 id;
>                          u16 len;
>                          u16 zerocopy:1;
>                          u32 bytelen;
>                  };
>          };
>
>          struct mmpin {
>                  struct user_struct *user;
>                  unsigned int num_pg;
>          } mmp;
> };
>
> Then you should be able to:
> - access ubuf_info->callback,
> - access the same field via ubuf_info->common.callback
> - declare variables as 'struct ubuf_info_commom' with appropriate
> contents.
>
> WDYT?

Interesting, I didn't think about struct_group, this would
let to split patches better and would limit non-core changes.
But if the plan is to convert the core helpers to
ubuf_info_common, than I think it's still messier than changing
ubuf providers only.

I can do the exercise, but I don't really see what is the goal.
Let me ask this, if we forget for a second how diffs look,
do you care about which pair is going to be in the end?
ubuf_info_common/ubuf_info vs ubuf_info/ubuf_info_msgzc?
Are there you concerned about naming or is there more to it?

--
Pavel Begunkov