[PATCH 4/9] General notification queue with user mmap()'able ring buffer [ver #5]

[Greg Kroah-Hartman]

On Fri, Jun 28, 2019 at 04:49:10PM +0100, David Howells 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);
> 
> The ring is divided into 8-byte slots.  Entries written into the ring are
> variable size and can use between 1 and 63 slots.  A special entry is
> maintained in the first two slots of the ring that contains the head and
> tail pointers.  This is skipped when the ring wraps round.  Note that
> multislot entries, therefore, aren't allowed to be broken over the end of
> the ring, but instead "skip" entries are inserted to pad out the buffer.
> 
> Each entry has a 1-slot header that describes it:
> 
> 	struct watch_notification {
> 		__u32	type:24;
> 		__u32	subtype:8;
> 		__u32	info;
> 	};
> 
> The type indicates the source (eg. mount tree changes, superblock events,
> keyring changes, block layer events) and the subtype indicates the event
> type (eg. mount, unmount; EIO, EDQUOT; link, unlink).  The info field
> indicates a number of things, including the entry length, an ID assigned to
> a watchpoint contributing to this buffer, type-specific flags and meta
> flags, such as an overrun indicator.
> 
> Supplementary data, such as the key ID that generated an event, are
> attached in additional slots.
> 
> Signed-off-by: David Howells <dhowells at redhat.com>

I don't know if I mentioned this before, but your naming seems a bit
"backwards" from other subsystems. Should "watch_queue" always be the
prefix, instead of a mix of prefix/suffix usage?

Anyway, your call, it's your code :)

Reviewed-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>