Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().

[Minchan Kim <minchan@kernel.org>]

On Thu, Feb 20, 2020 at 10:36:38AM -0800, Brian Geffon wrote:
> Hi Minchan,
> 
> > And here we got error if the addr is in non-anonymous-private vma so the
> > syscall will fail but old vma is gone? I guess it's not your intention?
> 
> This is exactly what happens today in several situations, because
> vma_to_resize is called unconditionally. For example if the old vma
> has VM_HUGETLB and old_len < new_len it would have unmapped a portion
> and then in vma_to_resize returned -EINVAL, similarly when old_len = 0
> with a non-sharable mapping it will have called do_munmap only to fail
> in vma_to_resize, if the vma has VM_DONTEXPAND set and you shrink the
> size with old_len < new_len it would return -EFAULT after having done
> the unmap on the decreased portion. So I followed the pattern to keep
> the change simple and maintain consistency with existing behavior.

Fair enough. It seems to be very old existing behavior but man page
never mention about it. :(

> 
> But with that being said, Kirill made the point that resizing a VMA
> while also using MREMAP_DONTUNMAP doesn't have any clear use case and
> I agree with that, I'm unable to think of a situation where you'd want
> to resize a VMA and use MREMAP_DONTUNMAP. So I'm tempted to mail a new
> version which returns -EINVAL if old_len != new_len that would resolve
> this concern here as nothing would be unmapped ever at the old
> position add it would clean up the change to very few lines of code.
> 
> What do you think?

Agreed. That makes code more simple/clean.

Thanks!