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

[Pavel Emelyanov]

On 12/15/2015 08:55 PM, Stanislav Kinsburskiy wrote:
> There is something, I would like to discuss.
> Currently, only _write_ end as O_DIRECT flag. Criu uses _read_ end to 
> tee data from it to local pipe. There is a problem, _how_ to discover, 
> whether pipe is "packetized" or not. This knowledge is required, because 
> splice() can be used only for non-packetized pipes (otherwise all 
> individual packets will be merged to one in image file).
> 
> But, I'm afraid, it's not enough... Because "packetized" pipe mode is 
> not something stable, because it's represented by file flag. Regular 
> pipe can be reopened with O_DIRECT flag or vise versa, but packets, 
> which were sent, _when_ pipe was _"packetized"_, are already _marked_ as 
> _"non-mergeable"_.
> So, probably, in generic case, it's not enough to create a pipe on 
> restore, fill it with data and set correct file flags, because pipe can 
> contain both "packets" and regular content.

I wouldn't say it's typical use for pipes. Can we support only those
that have all ends in the same state -- either packetized or not?

> With dump situation is not better, or maybe even worse.
> And dumping and restoring of such a pipe become a tricky operation.
> 
> Is there any ideas, how this problem should be handled?
> 
> 15.12.2015 11:57, Stanislav Kinsburskiy пишет:
>> This patch set adds support for pipes, working in packetized mode.
>>
>> Important Notes (kernel related):
>>
>> 1) Unfortunatelly, only write end is marked awith O_DIRECT in upstream kernel.
>> While this patch set relies on the fact, that read end is also marked with
>> If there is a simple way to find write end by read end, then would be nice to
>> avoid this kernel patching.
>>
>> 2) Splice engine is broken for packetized pipes. This have to fixed in
>> upstream. I'll provide patches soon.
> 
> .
>