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

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 <>

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