Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer [ver #13]

From: Miklos Szeredi
Date: Wed May 29 2019 - 03:59:15 EST

On Tue, May 28, 2019 at 5:10 PM David Howells <dhowells@xxxxxxxxxx> wrote:
> Implement a misc device that implements a general notification queue as a
> ring buffer that can be mmap()'d from userspace.
> The way this is done is:
> (1) An application opens the device and indicates the size of the ring
> buffer that it wants to reserve in pages (this can only be set once):
> fd = open("/dev/watch_queue", O_RDWR);
> ioctl(fd, IOC_WATCH_QUEUE_NR_PAGES, nr_of_pages);
> (2) The application should then map the pages that the device has
> reserved. Each instance of the device created by open() allocates
> separate pages so that maps of different fds don't interfere with one
> another. Multiple mmap() calls on the same fd, however, will all work
> together.
> page_size = sysconf(_SC_PAGESIZE);
> mapping_size = nr_of_pages * page_size;
> char *buf = mmap(NULL, mapping_size, PROT_READ|PROT_WRITE,
> MAP_SHARED, fd, 0);

Would it make sense to use relayfs for the implementation of the
mapped ring buffer?