Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled()

[David Hildenbrand (Arm)]

On 2/11/26 17:16, Ackerley Tng wrote:
> "David Hildenbrand (Arm)" <david@xxxxxxxxxx> writes:
>
> > On 2/11/26 01:58, Ackerley Tng wrote:
> > > Ackerley Tng <ackerleytng@xxxxxxxxxx> writes:
> > >
> > >
> > > On 6.1.162, secretmem VMAs are skipped since secretmem VMAs are not
> > > anonymous [2].
> > >
> > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/huge_memory.c?h=v6.1.162#n135
> > >
> > > Same for 6.6.123 [3].
> > >
> > > [3] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/huge_memory.c?h=v6.6.123#n125
> > >
> > > It breaks in 6.12.69 [4].
> >
> >
> > Do you have a reproducer? If so, which behavior does it trigger?
> >
> > I would assume that we would suddenly have secretmem pages (THP) that
> > have a directmap. Or some page copy would crash the kernel.
>
> Is there a good way to verify from userspace that the directmap hasn't
> been restored? Should I use CONFIG_PTDUMP_DEBUGFS?

Anything that uses GUP must fail on that secretmem memory. Like doing an 
O_DIRECT read/write or using vmsplice.

--
Cheers,

David