[CRIU] [RFC] Migration of pipes with O_DIRECT flag.

[Pavel Emelyanov]

On 12/11/2015 03:12 PM, Stanislav Kinsburskiy wrote:
> Good day,
> 
> MIgration of pipes with O_DIRECT is not supported yet.

Ack. Not supported.

> However, AutoFS control pipe is marked with O_DIRECT flag. Migration of 
> the content of control pipe is not supported, because it requires 
> migration of some kernel state.
> What is even more important, is that original pipe, created my process 
> and provided to kernel, can doesn't have this flag: it's set by kernel 
> on AutoFS mount unconditionally.
> Thus, it's enough just to remove check for O_DIRECT to allow AutoFS 
> migration.
> 
> However, it's not a generic solution and such a pipe should me migrated 
> properly.
> Major difference between ordinary pipe and O_DIRECT-one (within criu 
> scope), is that splice can't be used to dumping pipe data, because in 
> case of O_DIRECT flag pipe becomes a kind of queue, where individual 
> writes are not merged and have to be delivered to reader with it's 
> original size one per each read request.

O'RLY? Can you point the place in the kernel that prevents us from tee-ing
the pipe data as we do now?

> Thus, migration of data from pipe with O_DIRECT have to be done differently.
> Possible solutions are:
> 1) Modify existent pipe entry image to contain a header with data size 
> after it. This will allow to fill image with array of header-data 
> objects. Not backward compatible.

Pipe entry shouldn't be modified. There's a flags field that can just
carry the O_DIRECT one. If you want more explicit thing add optional
bool packetized field. Backward compatible.

The better question is how to store packets in pipe-data images. The
obvious solution is to keep several pipe_data_entry-s. At the first
glace it would work.

> 2) Add one more field to pipe entry (some kind of sequence number) and 
> create a bunch of pipe entry files with equal id and other attributes, 
> but different sequence numbers. Since it's a new field in protobuf, is 
> should be backward compatible.
> 3) Create another protobuf object for pipes with O_DIRECT.
> 
> Maybe, there are more and better, than these.
> 
> Would be great to hear any ideas.
> 
> Sincerely,
> Stanislav Kinsburskiy
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