Re: [PATCH] Alternate futex non-page-pinning and COW fix

[Linus Torvalds]

On Wed, 3 Sep 2003, Jamie Lokier wrote:
>
> Hugh Dickins wrote:
> > > Good question.  No kernel code seems to check VM_MAYSHARE - the one to
> > > check is VM_SHARED.
> > 
> > Observe fs/procfs/task_mmu.c show_map checking VM_MAYSHARE for 's'.
> > Observe mm/mmap.c do_mmap_pgoff vm_flags &= ~(VM_MAYWRITE | VM_SHARED).
> > VM_MAYSHARE reflects whether user chose MAP_SHARED, VM_SHARED may not.
> 
> Hugh, thank you.  In the case of futex.c, either flag could be used to
> mean "is this a shared" mapping, and each choice has a different
> user-visible meaning.

Actually: the VM_SHARED flag will never change, so testing VM_SHARED is 
actually the _right_ thing from a mm perspective.

The only person who should ever test VM_MAYSHARE is somebody who does
reporting back to user space: VM_MAYSHARE basically ends up meaning "the
user _asked_ for a shared mapping". While "VM_SHARED" means "this mapping
can actually contain a shared dirty page".

The VM itself should only ever care about VM_SHARED. Because that's the 
only bit that has real semantic meaning.

			Linus

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