On Wed, Mar 27, 2024 at 12:51:51PM +0100, David Hildenbrand wrote:
On 27.03.24 11:23, Stefano Garzarella wrote:
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?
I'd handle both like we handle files. But I understand now that you
really just want to "simulate" memfd.
Right, maybe I should write that in the commit message and
documentation.
- call shm_unlink() only at object deallocation?
Right, we don't really do that for ordinary files, they keep existing.
We only have the "discard-data" property to free up memory.
For memfd, it just happens "automatically". They cannot be looked up
by name, and once the last user closes the fd, it gets destroyed.
Yep, I see.
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.
Okay, so for now you really only want an anonymous fd that behaves like
a memfd, got it.
Likely we should somehow fail if the "POSIX shared memory object"
already exists. Hmm.
`O_CREAT | O_EXCL` should provide just this behavior.
From shm_open(3) manpage:
O_EXCL If O_CREAT was also specified, and a shared memory object
with the given name already exists, return an error. The
check for the existence of the object, and its creation if
it does not exist, are performed atomically.
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.
If it's really just an "internal UUID", as we'll remove the file using
shm_unlink(), then the name in fact shouldn't matter and this would be
fine. Correct?
Right. It's a name that will only "appear" in the system for a very
short time since we call shm_unlink() right after shm_open(). So I just
need the unique name to prevent several QEMUs launched at the same time
from colliding with the name.
So I assume if we ever want to have non-anonymous fds here, we'd pass
in a new property "name", and change the logic how we open/unlink.
Makes sense.
Exactly! I can clarify this in the commit description as well.
Thanks for the helpful comments!
If there is anything I need to change for v3, please tell me ;-)
Stefano
