On Mon, Apr 11, 2022 at 06:10:23PM +0300, Kirill A. Shutemov wrote:
> On Thu, Mar 10, 2022 at 10:08:59PM +0800, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > Introduce a new memfd_create() flag indicating the content of the
> > created memfd is inaccessible from userspace through ordinary MMU
> > access (e.g., read/write/mmap). However, the file content can be
> > accessed via a different mechanism (e.g. KVM MMU) indirectly.
> >
> > It provides semantics required for KVM guest private memory support
> > that a file descriptor with this flag set is going to be used as the
> > source of guest memory in confidential computing environments such
> > as Intel TDX/AMD SEV but may not be accessible from host userspace.
> >
> > Since page migration/swapping is not yet supported for such usages
> > so these pages are currently marked as UNMOVABLE and UNEVICTABLE
> > which makes them behave like long-term pinned pages.
> >
> > The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
> > also impossible for a memfd created with this flag.
> >
> > At this time only shmem implements this flag.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> > include/linux/shmem_fs.h | 7 +++++
> > include/uapi/linux/memfd.h | 1 +
> > mm/memfd.c | 26 +++++++++++++++--
> > mm/shmem.c | 57 ++++++++++++++++++++++++++++++++++++++
> > 4 files changed, 88 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> > index e65b80ed09e7..2dde843f28ef 100644
> > --- a/include/linux/shmem_fs.h
> > +++ b/include/linux/shmem_fs.h
> > @@ -12,6 +12,9 @@
> >
> > /* inode in-kernel data */
> >
> > +/* shmem extended flags */
> > +#define SHM_F_INACCESSIBLE 0x0001 /* prevent ordinary MMU access (e.g. read/write/mmap) to file content */
> > +
> > struct shmem_inode_info {
> > spinlock_t lock;
> > unsigned int seals; /* shmem seals */
> > @@ -24,6 +27,7 @@ struct shmem_inode_info {
> > struct shared_policy policy; /* NUMA memory alloc policy */
> > struct simple_xattrs xattrs; /* list of xattrs */
> > atomic_t stop_eviction; /* hold when working on inode */
> > + unsigned int xflags; /* shmem extended flags */
> > struct inode vfs_inode;
> > };
> >
>
> AFAICS, only two bits of 'flags' are used. And that's very strange that
> VM_ flags are used for the purpose. My guess that someone was lazy to
> introduce new constants for this.
>
> I think we should fix this: introduce SHM_F_LOCKED and SHM_F_NORESERVE
> alongside with SHM_F_INACCESSIBLE and stuff them all into info->flags.
> It also makes shmem_file_setup_xflags() go away.
Did a quick search and sounds we only use SHM_F_LOCKED/SHM_F_NORESERVE and
that definitely don't have to be VM_ flags.
Chao
>
> --
> Kirill A. Shutemov
