On Mon, 1 Aug 2005, Linus Torvalds wrote:
>
> Of course, if VM_MAYWRITE is not set, you could just convert it silently
> to a MAP_PRIVATE at the VM level (that's literally what we used to do,
> back when we didn't support writable shared mappings at all, all those
> years ago), so at least now the COW behaviour would match the vma_flags.
Heh. I just checked. We still do exactly that:
if (!(file->f_mode & FMODE_WRITE))
vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
some code never dies ;)
However, we still set the VM_MAYSHARE bit, and thats' the one that
mm/rmap.c checks for some reason. I don't see quite why - VM_MAYSHARE
doesn't actually ever do anything else than make sure that we try to
allocate a mremap() mapping in a cache-coherent space, I think (ie it's a
total no-op on any sane architecture, and as far as rmap is concerned on
all of them).
Linus
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