Use of qemu_memfd_check() in system/physmem.c (was: [PATCH 0/7] backends/hostmem: Make QOM types and properties unconditional)

[Markus Armbruster <armbru@redhat.com>]

After this series, there is just one use of qemu_memfd_check() outside
tests/ left:

    /*
     * Create MAP_SHARED RAMBlocks by mmap'ing a file descriptor, so it can be
     * shared with another process if CPR is being used.  Use memfd if available
     * because it has no size limits, else use POSIX shm.
     */
    static int qemu_ram_get_shared_fd(const char *name, bool *reused, Error **errp)
    {
        int fd = cpr_find_fd(name, 0);

        if (fd >= 0) {
            *reused = true;
            return fd;
        }

--->    if (qemu_memfd_check(0)) {
            fd = qemu_memfd_create(name, 0, 0, 0, 0, errp);
        } else {
            fd = qemu_shm_alloc(0, errp);
        }

        if (fd >= 0 && !cpr_save_fd(name, 0, fd, errp)) {
            close(fd);
            fd = -1;
        }
        *reused = false;
        return fd;
    }

We test whether memfd_create("test", MFD_CLOEXEC) succeeds to decide
whether to use memfd_create(name, MFD_CLOEXEC) or POSIX shared memory.
This is anti-pattern "predict which method should succed, then try that
method".  A better way is "try methods one after the other until one
succeeds".

Doing that here would require refactoring, because both
qemu_memfd_create() and qemu_shm_alloc() do additional work that can
also fail: ftruncate().

Doubt: qemu_memfd_create(0) caches its result[*].  Is that a mere
optimization, or does something depend on "we use the same method
always"?

I don't plan to mess with this myself, I'm merely reporting something I
found.


[*] Only when the argument is 0.