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Message-ID: <3d64246c-e907-44bd-b098-a5373740347b@kernel.dk>
Date: Tue, 1 Apr 2025 13:59:24 -0600
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Subject: Re: [PATCH v2] io_uring/zcrx: return early from io_zcrx_recv_skb if
readlen is 0
To: David Wei <dw@davidwei.uk>, io-uring@vger.kernel.org
Cc: Pavel Begunkov <asml.silence@gmail.com>
References: <20250401195355.1613813-1-dw@davidwei.uk>
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From: Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20250401195355.1613813-1-dw@davidwei.uk>
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On 4/1/25 1:53 PM, David Wei wrote:
> When readlen is set for a recvzc request, tcp_read_sock() will call
> io_zcrx_recv_skb() one final time with len == desc->count == 0. This is
> caused by the !desc->count check happening too late. The offset + 1 !=
> skb->len happens earlier and causes the while loop to continue.
>
> Fix this in io_zcrx_recv_skb() instead of tcp_read_sock(). Return early
> if len is 0 i.e. the read is done.
>
> Changes in v2:
> --------------
> * Add Fixes tag
> * Return 0 directly
> * Add comment explaining why the !len check is needed
>
> Fixes: 6699ec9a23f8 ("io_uring/zcrx: add a read limit to recvzc requests")
> Signed-off-by: David Wei <dw@davidwei.uk>
> ---
> io_uring/zcrx.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index 9c95b5b6ec4e..2c8b29c745c5 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -818,6 +818,13 @@ io_zcrx_recv_skb(read_descriptor_t *desc, struct sk_buff *skb,
> int ret = 0;
>
> len = min_t(size_t, len, desc->count);
> + /* __tcp_read_sock() always calls io_zcrx_recv_skb one last time, even
> + * if desc->count is already 0. This is caused by the if (offset + 1 !=
> + * skb->len) check. Return early in this case to break out of
> + * __tcp_read_sock().
> + */
> + if (!len)
> + return 0;
> if (unlikely(args->nr_skbs++ > IO_SKBS_PER_CALL_LIMIT))
> return -EAGAIN;
Bad comment format, but I can fix it up while applying. Thanks!
--
Jens Axboe