Kdbus meets linux-kernel [LWN.net]

Kdbus meets linux-kernel

Posted Nov 5, 2014 2:01 UTC (Wed) by riking (subscriber, #95706)
Parent article: Kdbus meets linux-kernel

> Messages can pass file descriptors between processes; the passing of sealed files and memfds is also supported.
> ...
> Kdbus is intended to be fast with both large and small messages. For the largest of messages, zero-copy transfer between processes is supported. Experience has shown, though, that a message must be about 512KB or larger before page-mapping tricks become cheaper than just copying the data.

To clarify, the zero-copy mechanism *is* sealed memfd file descriptor passing.

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Kdbus meets linux-kernel

Posted Nov 5, 2014 14:05 UTC (Wed) by cesarb (subscriber, #6266) [Link] (1 responses)

So, it's the sender's decision whether to zero-copy, instead of the kernel's?

Kdbus meets linux-kernel

Posted Nov 5, 2014 14:41 UTC (Wed) by dvdhrm (subscriber, #85474) [Link]

> So, it's the sender's decision whether to zero-copy, instead of the kernel's?

The sender needs to provide data in a way suitable for zero-copy. For kdbus, this means it has to be in a memfd. Whether the kernel passes this data around via the fd, or whether it inlines it into the receiver's buffer, is *not* controlled by the sender. So far, kdbus always passes data the same way it received it, though.