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Date:   Fri, 21 Oct 2022 12:09:09 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Stefan Metzmacher <metze@...ba.org>,
        io-uring <io-uring@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>,
        Dylan Yudaken <dylany@...com>
Subject: Re: IORING_SEND_NOTIF_REPORT_USAGE (was Re: IORING_CQE_F_COPIED)

On 10/21/22 10:36, Stefan Metzmacher wrote:
> Hi Pavel,
[...]
>> Right, I'm just tired of back porting patches by hand :)
> 
> ok, I just assumed it would be 6.1 only.

I'm fine with 6.1 only, it'd make things easier. I thought from
your first postings you wanted it 6.0. Then we don't need to care
about the placing of the copied/used flags.

>>> Otherwise we could have IORING_CQE_F_COPIED by default without opt-in
>>> flag...
> 
> Do you still want an opt-in flag to get IORING_CQE_F_COPIED?
> If so what name do you want it to be?

Ala a IORING_SEND_* flag? Yes please.

*_REPORT_USAGE was fine but I'd make it IORING_SEND_ZC_REPORT_USAGE.
And can be extended if there is more info needed in the future.

And I don't mind using a bit in cqe->res, makes cflags less polluted.


>>>>> +static void io_uring_tx_zerocopy_callback_report_usage(struct sk_buff *skb,
>>>>> +                            struct ubuf_info *uarg,
>>>>> +                            bool success)
>>>>> +{
>>>>> +    struct io_notif_data *nd = container_of(uarg, struct io_notif_data, uarg);
>>>>> +
>>>>> +    if (success && !nd->zc_used && skb)
>>>>> +        nd->zc_used = true;
>>>>> +    else if (unlikely(!success && !nd->zc_copied))
>>>>> +        nd->zc_copied = true;
>>>>
>>>> It's fine but racy, so let's WRITE_ONCE() to indicate it.
>>>
>>> I don't see how this could be a problem, but I can add it.
>>
>> It's not a problem, but better to be a little be more explicit
>> about parallel writes.
> 
> ok.
> 
>>>>> diff --git a/io_uring/notif.h b/io_uring/notif.h
>>>>> index 5b4d710c8ca5..5ac7a2745e52 100644
>>>>> --- a/io_uring/notif.h
>>>>> +++ b/io_uring/notif.h
>>>>> @@ -13,10 +13,12 @@ struct io_notif_data {
>>>>>       struct file        *file;
>>>>>       struct ubuf_info    uarg;
>>>>>       unsigned long        account_pages;
>>>>> +    bool            zc_used;
>>>>> +    bool            zc_copied;
>>>>
>>>> IIRC io_notif_data is fully packed in 6.1, so placing zc_{used,copied}
>>>> there might complicate backporting (if any). We can place them in io_kiocb
>>>> directly and move in 6.2. Alternatively account_pages doesn't have to be
>>>> long.
>>>
>>> As far as I can see kernel-dk-block/io_uring-6.1 alread has your
>>> shrink patches included...
>>
>> Sorry, I mean 6.0
> 
> So you want to backport to 6.0?
> 
> Find the current version below, sizeof(struct io_kiocb) will grow from
> 3*64 + 24 to 3*64 + 32 (on x64_64) to it stays within 4 cache lines.
> 
> I tried this first:
> 
> union {
>    u8 iopoll_completed;
>    struct {
>      u8 zc_used:1;
>      u8 zc_copied:1;
>    };
> };
> 
> But then WRITE_ONCE() complains about a bitfield write.

rightfully so, it can't be a bitfield as it would be racy
and not only in theory this time.


> So let me now about the opt-in flag and I'll prepare real commits
> including a patch that moves from struct io_kiocb to struct io_notif_data
> on top.

Yeah, better to be opt-in, but apart from it and comments above
looks good.


> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
> index f5b687a787a3..189152ad78d6 100644
> --- a/include/linux/io_uring_types.h
> +++ b/include/linux/io_uring_types.h
> @@ -515,6 +515,9 @@ struct io_kiocb {
>       u8                opcode;
>       /* polled IO has completed */
>       u8                iopoll_completed;
> +    /* these will be moved to struct io_notif_data in 6.1 */
> +    bool                zc_used;
> +    bool                zc_copied;
>       /*
>        * Can be either a fixed buffer index, or used with provided buffers.
>        * For the latter, before issue it points to the buffer group ID,
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index ab7458033ee3..738d6234d1d9 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -350,6 +350,7 @@ struct io_uring_cqe {
>   #define IORING_CQE_F_MORE        (1U << 1)
>   #define IORING_CQE_F_SOCK_NONEMPTY    (1U << 2)
>   #define IORING_CQE_F_NOTIF        (1U << 3)
> +#define IORING_CQE_F_COPIED        (1U << 4)
> 
>   enum {
>       IORING_CQE_BUFFER_SHIFT        = 16,
> diff --git a/io_uring/notif.c b/io_uring/notif.c
> index e37c6569d82e..033aca064b10 100644
> --- a/io_uring/notif.c
> +++ b/io_uring/notif.c
> @@ -18,6 +18,10 @@ static void __io_notif_complete_tw(struct io_kiocb *notif, bool *locked)
>           __io_unaccount_mem(ctx->user, nd->account_pages);
>           nd->account_pages = 0;
>       }
> +
> +    if (notif->zc_copied || !notif->zc_used)
> +        notif->cqe.flags |= IORING_CQE_F_COPIED;
> +

As discussed above, should depend on IORING_SEND_ZC_REPORT_USAGE

>       io_req_task_complete(notif, locked);
>   }
> 
> @@ -28,6 +32,11 @@ static void io_uring_tx_zerocopy_callback(struct sk_buff *skb,
>       struct io_notif_data *nd = container_of(uarg, struct io_notif_data, uarg);
>       struct io_kiocb *notif = cmd_to_io_kiocb(nd);
> 
> +    if (success && !notif->zc_used && skb)
> +        WRITE_ONCE(notif->zc_used, true);
> +    else if (!success && !notif->zc_copied)
> +        WRITE_ONCE(notif->zc_copied, true);
> +
>       if (refcount_dec_and_test(&uarg->refcnt)) {
>           notif->io_task_work.func = __io_notif_complete_tw;
>           io_req_task_work_add(notif);
> @@ -55,6 +64,7 @@ struct io_kiocb *io_alloc_notif(struct io_ring_ctx *ctx)
>       nd->account_pages = 0;
>       nd->uarg.flags = SKBFL_ZEROCOPY_FRAG | SKBFL_DONT_ORPHAN;
>       nd->uarg.callback = io_uring_tx_zerocopy_callback;
> +    notif->zc_used = notif->zc_copied = false;
>       refcount_set(&nd->uarg.refcnt, 1);
>       return notif;
>   }
> 

-- 
Pavel Begunkov

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