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

On 9/27/22 21:23, Paolo Abeni wrote:

On Tue, 2022-09-27 at 21:17 +0100, Pavel Begunkov wrote:

On 9/27/22 20:59, Paolo Abeni wrote:

On Tue, 2022-09-27 at 19:48 +0100, Pavel Begunkov wrote:

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?


Uhm... I proposed this initially with the goal of remove non fuctional
changes from a patch that was hard to digest for me (4/4). So it's
about diffstat to me ;)


Ah, got it

On the flip side the change suggested would probably not be as
straighforward as I would hope for.

ubuf_info_common/ubuf_info vs ubuf_info/ubuf_info_msgzc?


The specific names used are not much relevant.

Are there you concerned about naming or is there more to it?


I feel like this series is potentially dangerous, but I could not spot
bugs into the code. I would have felt more relaxed eariler in the devel
cycle.


union {
        struct {
                unsigned long desc;
                void *ctx;
        };
        struct {
                u32 id;
                u16 len;
                u16 zerocopy:1;
                u32 bytelen;
        };
};


btw, nobody would frivolously change ->zerocopy anyway as it's
in a union. Even without the series we're absolutely screwed
if someone does that. If anything it adds a way to get rid of it:

1) Make vhost and xen use their own structures with right types.
2) kill unused struct {ctx, desc} for MSG_ZEROCOPY


Ok, the above sounds reasonable. Additionally I've spent the last
surviving neuron on my side to on this series, and it looks sane, so...

Acked-by: Paolo Abeni <pabeni@xxxxxxxxxx>

Great, thanks for taking a look! -- Pavel Begunkov

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