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Message-ID: <3c1a39f0-a6d7-41af-8235-af299f164633@kernel.dk>
Date: Tue, 1 Apr 2025 13:12:22 -0600
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Subject: Re: [PATCH] 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: <20250401182813.1115909-1-dw@davidwei.uk>
<e89aef50-7364-4ab9-9582-aef6aec8cffb@kernel.dk>
<20e4d11f-688f-46de-9094-765073f0dc41@davidwei.uk>
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From: Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20e4d11f-688f-46de-9094-765073f0dc41@davidwei.uk>
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On 4/1/25 1:10 PM, David Wei wrote:
> On 2025-04-01 11:56, Jens Axboe wrote:
>> On 4/1/25 12:28 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.
>>
>> Needs a Fixes tag, which looks like it should be:
>>
>> Fixes: 6699ec9a23f8 ("io_uring/zcrx: add a read limit to recvzc requests")
>>
>> ?
>
> Sorry I missed that, will add the tag.
>
>>
>>> Signed-off-by: David Wei <dw@davidwei.uk>
>>> ---
>>> io_uring/zcrx.c | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
>>> index 9c95b5b6ec4e..d1dd25e7cf4a 100644
>>> --- a/io_uring/zcrx.c
>>> +++ b/io_uring/zcrx.c
>>> @@ -818,6 +818,8 @@ io_zcrx_recv_skb(read_descriptor_t *desc, struct sk_buff *skb,
>>> int ret = 0;
>>>
>>> len = min_t(size_t, len, desc->count);
>>> + if (!len)
>>> + goto out;
>>
>> just return 0 here? Jumping to out would make more sense if there
>> are things to fixup/account at this point, but it's just going
>> to find offset == start_off and return 'ret', which is 0 anyway.
>>
>
> Makes sense, yeah. I'll return 0 here early.
Probably augment that with an unlikely() as well for v2, it's definitely
an error/unexpected case.
--
Jens Axboe