Re: [PATCH v3 03/26] mm: introduce AS_NO_DIRECT_MAP

[Yosry Ahmed]

On Tue, Oct 13, 2026 at 9:06 AM Takahiro Itazuri <itazur@xxxxxxxxxx> wrote:
>
> On Tue, 13 Oct 2026 08:36:54 -0700, Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> > > 3. ALLOC_UNMAPPED and the mermap
> > >
> > > AS_NO_DIRECT_MAP itself works without them, by zapping the direct map
> > > of each folio. However, guest_memfd will eventually need a temporary
> > > kernel mapping to populate guest memory via write() [2], because
> > > folios are already unmapped once they are added to the page cache.
> [...]
> >
> > FWIW, I think the mermap can still be used without ALLOC_UNMAPPED, and
> > I did have a newer version of the x86 patches repurposing the LDT area
> > for the mermap, and simplifying the mermap implementation a bit. I can
> > share them privately if you want to take a look.
>
> Yes please, that would be very helpful. Thanks!
>
> Agreed that the mermap can be used without ALLOC_UNMAPPED. To give a
> bit more context from the guest_memfd side: I can't say anything
> definite until I've gone through the use cases, but as far as I can
> see now, guest_memfd needs the mermap only for write() support.
> write() is an optimization for populating guest memory (e.g. when
> restoring a snapshot) and isn't functionally required, because
> userspace can also populate guest memory through its own mapping of
> guest_memfd. So I currently plan to keep write() support, and with it
> the mermap, out of the guest_memfd direct map removal series itself.
> Since ALLOC_UNMAPPED is the main user of the mermap, it may make more
> sense for the two to land together later.
>
> One thing I'd like to look into is arm64. Firecracker also runs on
> arm64, so write() needs to work there too, but the mermap currently
> relies on the x86 mm-local region, and it sounds like your newer
> version relies on the LDT area. Kevin, do you happen to have any
> ideas on how something similar could be done on arm64?

Small correction: Brendan's version already reworked the LDT area, I
just reshuffled things and simplified the implementation.

>
> Thanks,
> Takahiro