Re: [PATCH v5 01/13] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Kirill A. Shutemov]

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.

-- 
 Kirill A. Shutemov