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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 29 May 2019 09:55:38 +0200
Message-ID: <CAJfpeguPTQ00zVjpwVQ4R8mEqE3aijCzNMAz6Wvr56xE-jfJag@mail.gmail.com>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring
 buffer [ver #13]
To:     David Howells <dhowells@redhat.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, Ian Kent <raven@themaw.net>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Miklos Szeredi <mszeredi@redhat.com>
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On Tue, May 28, 2019 at 5:10 PM David Howells <dhowells@redhat.com> 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?

Thanks,
Miklos