Re: [PATCH for-9.1 v2 09/11] hostmem: add a new memory backend based on POSIX shm_open()

[Stefano Garzarella]

On Tue, Mar 26, 2024 at 03:45:52PM +0100, David Hildenbrand wrote:
> > +    mode = 0;
> > +    oflag = O_RDWR | O_CREAT | O_EXCL;
> > +    backend_name = host_memory_backend_get_name(backend);
> > +
> > +    /*
> > +     * Some operating systems allow creating anonymous POSIX shared memory
> > +     * objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
> > +     * defined by POSIX, so let's create a unique name.
> > +     *
> > +     * From Linux's shm_open(3) man-page:
> > +     *   For  portable  use,  a shared  memory  object should be identified
> > +     *   by a name of the form /somename;"
> > +     */
> > +    g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%s", getpid(),
> > +                    backend_name);
>
> Hm, shouldn't we just let the user specify a name, and if no name was 
> specified, generate one ourselves?

I thought about it and initially did it that way, but then some problems 
came up so I tried to keep it as simple as possible for the user and for 
our use case (having an fd associated with memory and sharing it with 
other processes).

The problems I had were:

- what mode_t to use if the object does not exist and needs to be 
  created?

- exclude O_EXCL if the user passes the name since they may have already 
  created it?

- call shm_unlink() only at object deallocation?

So I thought that for now we only support the "anonymous" mode, and if 
in the future we have a use case where the user wants to specify the 
name, we can add it later.

That said, if you think it's already useful from the beginning, I can 
add the name as an optional parameter.

> I'm also not quite sure if "host_memory_backend_get_name()" should be 
> used for the purpose here.

What problem do you see? As an alternative I thought of a static 
counter.

Thanks,
Stefano