Pipe + splice problems

Hi,

While working on the checkpoint-restore project and looking at what other Parallels people do with FUSE, I have met several drawbacks of pipes. I admit, that most of them are well known for years, but would like to summarize them and refresh the topic.

So the major problems are

* Pipe as page buffer lacks the random access feature

In FUSE-based FS-es like CEPH or Gluster with complex internal data flows it's common case when data arrive in pipe, but FS is willing to forward the pages from pipe tail into one descriptor while keeping those from head in memory waiting for unblocked another. Pipes do not allow this.

* Pipe's locking is one big mutex

For pipes with more than one page in buffers this results in readers-vs-writers contention and, subsequently, pipe works slower than a UNIX socket

* The pipe->mem vmsplice always copies data

There are cases in C/R when we have pages in pipes that cane be mapped in tasks' address spaces, but vmslice doesn't allow for that.

* No pipe -> AIO splicing

The pipe -> FS always goes through page cache, while AIO is more preferable in some scenarios

* No fallocate analogue for pipe

People report many calls to __alloc_page in profiling logs when heavily working with pipes

And a couple of minor issues

* Pipe requires 2 FDs to work with

With this using pipe as generic page-buffer is difficult due to nr_files limitation.

* No sendpage for UNIX sockets results in pipe->unix data copy

Thanks,
Pavel