Re: [PATCH v2] mm: Add MREMAP_DONTUNMAP to mremap().

[Brian Geffon <bgeffon@google.com>]

Hi Florian,
copy_vma will make a copy of the existing VMA leaving the old VMA
unchanged, so the source keeps its existing protections, this is what
makes it very useful along with userfaultfd.

Thanks,
Brian


On Mon, Jan 27, 2020 at 2:13 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Brian Geffon:
>
> > When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> > set, the source mapping will not be removed. Instead it will be
> > cleared as if a brand new anonymous, private mapping had been created
> > atomically as part of the mremap() call.  If a userfaultfd was watching
> > the source, it will continue to watch the new mapping.  For a mapping
> > that is shared or not anonymous, MREMAP_DONTUNMAP will cause the
> > mremap() call to fail. MREMAP_DONTUNMAP implies that MREMAP_FIXED is
> > also used. The final result is two equally sized VMAs where the
> > destination contains the PTEs of the source.
>
> What will be the protection flags of the source mapping?  Will they
> remain unchanged?  Or PROT_NONE?
>
> Thanks,
> Florian
>