[CRIU] [PATCH 0/5] pipes: support for packetized mode (with O_DIRECT)

[Stanislav Kinsburskiy]

21.12.2015 11:33, Pavel Emelyanov пишет:
> On 12/18/2015 05:25 PM, Stanislav Kinsburskiy wrote:
>
>>>> We can try to read many times with size, equal to pipe size.
>>>> If we will be able to get more, than one buffer, then it's packetized pipe.
>>>   From my perspective we don't have to distinguish packetized pipe from regular
>>> when dumping queueu. Just write the data in the available chinks into the
>>> pipe-date image and that's it.
>> Packetized pipe is expected to be read by chunks sizeof PIPE_BUF (==
>> PAGE_SIZE).
>> In case of packetized pipe read will return only one buffer.
>> In case of normal pipe it will return as many buffers, as fit into PIPE_BUF.
>>
>>>> If only one, then on restore:
>>>> 1) If it's only read end without write, we don't care. We open a regular
>>>> pipe and write this packet.
>>>> 2) If it's a full pipe, then we a have a write end somewhere with
>>>> O_DIRECT flag or without.
>>>>
>>>> Looks like this approach covers all the cases (except one very special,
>>>> which we can not take into account).
>>>>
>>>> But it means, that we have to get rid of splice/vmsplice:
>>>> 1) On dump, because we have to write all the packets individually.
>>>> 2) On restore, because vmsplice doesn't work with packetized pipes.
>>> BTW, I've glanced through the kernel and haven't found why splice doesn't
>>> work for packetized pipes. Can you point out one?
>> Sure.
>> 1) Splice pipe to image file on dump. In this case we loose all the
>> packets structure and having one single data chunk.
> Why? Doesn't splice respect packetized buffers?

Imagine, that we have two packets.
And you write them to a file with splice.
How do you know the sizes of them after?

>
>> 2) Vmsplice on restore. Packetized buffers have to be marked with
>> special flag, which vmsplice is not aware of. This leads to lsituation,
>> when all the packets they are returned as one single data chunk to a
>> pipe reader.
> I see.
>
> -- Pavel
>