Re: [PATCH] Lightweight userspace semaphores...

[Alan Cox]

> On Mon, 25 Feb 2002, Alan Cox wrote:
> >
> > As opposed to adding special cases to the kernel which are unswappable and
> > stand to tangle up bits of the generic vfs - eg we would have a vma with
> > a vm_file but that file would not be in the dcache ?
> 
> Why should they be unswappable?

Any kernel special cases it adds will be unswappable because they are in
kernel space (not the semaphores here - we want them to be swappable and
they can be)

> It's the same thing as giving a -1 to mmap. That doesn't make it
> unswappable.

When you create a shared mapping by passing -1 to mmap we do

        } else if (flags & MAP_SHARED) {
                error = shmem_zero_setup(vma);

shmem_zero_setup does

        file = shmem_file_setup("dev/zero", size);
        if (IS_ERR(file))
                return PTR_ERR(file);

        if (vma->vm_file)
                fput (vma->vm_file);
        vma->vm_file = file;
        vma->vm_ops = &shmem_vm_ops;

and we are back creating file names. Basically because a shared mmap in
Linux needs vma->vm_file, and vma->vm_file needs all the rest of the logic
behind it

Thats why I am saying that magic name picking is something that user space
might as well do for unnamed objects. We end up with names and vm_file
however we do it