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Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
To:     Kanchan Joshi <joshi.k@samsung.com>, viro@zeniv.linux.org.uk,
        bcrl@kvack.org
Cc:     willy@infradead.org, hch@infradead.org, Damien.LeMoal@wdc.com,
        asml.silence@gmail.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-aio@kvack.org,
        io-uring@vger.kernel.org, linux-block@vger.kernel.org,
        linux-api@vger.kernel.org, SelvaKumar S <selvakuma.s1@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>
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 <1595605762-17010-7-git-send-email-joshi.k@samsung.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <f5416bd4-93b3-4d14-3266-bdbc4ae1990b@kernel.dk>
Date:   Fri, 24 Jul 2020 10:29:19 -0600
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On 7/24/20 9:49 AM, Kanchan Joshi wrote:
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 7809ab2..6510cf5 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
>  	cqe = io_get_cqring(ctx);
>  	if (likely(cqe)) {
>  		WRITE_ONCE(cqe->user_data, req->user_data);
> -		WRITE_ONCE(cqe->res, res);
> -		WRITE_ONCE(cqe->flags, cflags);
> +		if (unlikely(req->flags & REQ_F_ZONE_APPEND)) {
> +			if (likely(res > 0))
> +				WRITE_ONCE(cqe->res64, req->rw.append_offset);
> +			else
> +				WRITE_ONCE(cqe->res64, res);
> +		} else {
> +			WRITE_ONCE(cqe->res, res);
> +			WRITE_ONCE(cqe->flags, cflags);
> +		}

This would be nice to keep out of the fast path, if possible.

> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 92c2269..2580d93 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -156,8 +156,13 @@ enum {
>   */
>  struct io_uring_cqe {
>  	__u64	user_data;	/* sqe->data submission passed back */
> -	__s32	res;		/* result code for this event */
> -	__u32	flags;
> +	union {
> +		struct {
> +			__s32	res;	/* result code for this event */
> +			__u32	flags;
> +		};
> +		__s64	res64;	/* appending offset for zone append */
> +	};
>  };

Is this a compatible change, both for now but also going forward? You
could randomly have IORING_CQE_F_BUFFER set, or any other future flags.
Layout would also be different between big and little endian, so not
even that easy to set aside a flag for this. But even if that was done,
we'd still have this weird API where liburing or the app would need to
distinguish this cqe from all others based on... the user_data? Hence
liburing can't do it, only the app would be able to.

Just seems like a hack to me.

-- 
Jens Axboe