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:

quoted

From: "Kirill A. Shutemov" <redacted>

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 <redacted>
Signed-off-by: Chao Peng <redacted>
---
 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