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From: Jens Axboe <[email protected]>
To: Pavel Begunkov <[email protected]>,
	Constantine Gavrilov <[email protected]>
Cc: [email protected], [email protected]
Subject: Re: [PATCH 1/2] io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly
Date: Wed, 23 Mar 2022 15:24:44 -0600	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 3/23/22 2:52 PM, Pavel Begunkov wrote:
> On 3/23/22 20:45, Constantine Gavrilov wrote:
>> On Wed, Mar 23, 2022 at 10:14 PM Pavel Begunkov <[email protected]> wrote:
>>>
>>> On 3/23/22 15:39, Jens Axboe wrote:
>>>> We currently don't attempt to get the full asked for length even if
>>>> MSG_WAITALL is set, if we get a partial receive. If we do see a partial
>>>> receive, then just note how many bytes we did and return -EAGAIN to
>>>> get it retried.
>>>>
>>>> The iov is advanced appropriately for the vector based case, and we
>>>> manually bump the buffer and remainder for the non-vector case.
>>>
>>> How datagrams work with MSG_WAITALL? I highly doubt it coalesces 2+
>>> packets to satisfy the length requirement (e.g. because it may move
>>> the address back into the userspace). I'm mainly afraid about
>>> breaking io_uring users who are using the flag just to fail links
>>> when there is not enough data in a packet.
>>>
>>> -- 
>>> Pavel Begunkov
>>
>> Pavel:
>>
>> Datagrams have message boundaries and the MSG_WAITALL flag does not
>> make sense there. I believe it is ignored by receive code on daragram
>> sockets. MSG_WAITALL makes sends only on stream sockets, like TCP. The
>> manual page says "This flag has  no  effect  for datagram sockets.".
> 
> Missed the line this in mans, thanks, and it's exactly as expected.
> The problem is on the io_uring side where with the patch it might
> blindly do a second call into the network stack consuming 2+ packets.

Right, it should not be applied for datagrams.

-- 
Jens Axboe


  reply	other threads:[~2022-03-23 21:24 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-23 15:39 [PATCHSET 0/2] Fix MSG_WAITALL for IORING_OP_RECV/RECVMSG Jens Axboe
2022-03-23 15:39 ` [PATCH 1/2] io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly Jens Axboe
2022-03-23 20:13   ` Pavel Begunkov
2022-03-23 20:15     ` Jens Axboe
2022-03-23 20:45     ` Constantine Gavrilov
2022-03-23 20:52       ` Pavel Begunkov
2022-03-23 21:24         ` Jens Axboe [this message]
2022-03-23 15:39 ` [PATCH 2/2] io_uring: add flag for disabling provided buffer recycling Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2022-03-23 22:41 [PATCHSET v2 0/2] Fix MSG_WAITALL for IORING_OP_RECV/RECVMSG Jens Axboe
2022-03-23 22:41 ` [PATCH 1/2] io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly Jens Axboe
2022-03-24  0:32   ` Stefan Metzmacher
2022-03-24  0:35     ` Jens Axboe

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