Re: [RFC PATCH 00/13] Introduce first class virtual address spaces

On Wed, Mar 15, 2017 at 12:44:47PM -0700, Till Smejkal wrote:

On Wed, 15 Mar 2017, Andy Lutomirski wrote:

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One advantage of VAS segments is that they can be globally queried by user programs
which means that VAS segments can be shared by applications that not necessarily have
to be related. If I am not mistaken, MAP_SHARED of pure in memory data will only work
if the tasks that share the memory region are related (aka. have a common parent that
initialized the shared mapping). Otherwise, the shared mapping have to be backed by a
file.

What's wrong with memfd_create()?

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VAS segments on the other side allow sharing of pure in memory data by
arbitrary related tasks without the need of a file. This becomes especially
interesting if one combines VAS segments with non-volatile memory since one can keep
data structures in the NVM and still be able to share them between multiple tasks.

What's wrong with regular mmap?

I never wanted to say that there is something wrong with regular mmap. We just
figured that with VAS segments you could remove the need to mmap your shared data but
instead can keep everything purely in memory.

Unfortunately, I am not at full speed with memfds. Is my understanding correct that
if the last user of such a file descriptor closes it, the corresponding memory is
freed? Accordingly, memfd cannot be used to keep data in memory while no program is
currently using it, can it? To be able to do this you need again some representation

I have a name for application-allocated kernel resources that persist
without a process holding a reference to them or a node in the
filesystem: a bug. See: sysvipc.

Rich