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

[Brian Geffon]

Hi Kirill,
I started a new thread https://lkml.org/lkml/2020/2/7/640 for my v4
patch. But I wanted to quickly address your comments. Regarding the
concern around the rmap, no changes actually need to be made. If we
were to unlink_anon_vma(vma) and then set vma->anon_vma = NULL, that
would be fine but then as soon as there was a fault the same anon_vma
would be attached since it's a private anonymous mapping. So there is
really nothing to do regarding the rmap.

I considered the two flag approach but since I could not come up with
a concrete use case of MREMAP_MUSTMOVE I decided to just leave the
single MREMAP_DONTUNMAP flag, the two flag approach would be only for
clarifying the operations so I'm not sure it's worth it. (Still trying
to come up with a better name). But I've attached a man page diff to
the latest patch.

Thanks,
Brian

On Mon, Feb 3, 2020 at 5:09 AM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>
> On Sun, Feb 02, 2020 at 05:17:53AM +0100, Brian Geffon wrote:
> > On Wed, Jan 29, 2020 at 11:46 AM Kirill A. Shutemov
> > <kirill@xxxxxxxxxxxxx> wrote:
> > > Any better options for the flag name? (I have none)
> >
> > The other option is that it's broken up into two new flags the first
> > MREMAP_MUSTMOVE which can be used regardless of whether or not you're
> > leaving the original mapping mapped. This would do exactly what it
> > describes: move the mapping to a new address with or without
> > MREMAP_FIXED, this keeps consistency with MAYMOVE.
> >
> > The second flag would be the new MREMAP_DONTUNMAP flag which requires
> > MREMAP_MUSTMOVE, again with or without MREMAP_FIXED.
> >
> > What are your thoughts on this?
>
> Sounds reasonable.
>
> MREMAP_DONTUNMAP doesn't really convey that you move pages to the new
> mapping, but leave empty mapping behind. But I guess there's only so much
> you can encode into the name. (Patch to the man page should do the rest)
>
> --
>  Kirill A. Shutemov