[RFC PATCH 4/5] mm/memfd_luo: Expand support beyond memfd to all generic shmem inodes

[David Matlack]

Relax the early validation checks in the Live Update preservation path
to allow general shmem-backed files, completely dropping the requirement
that the file be unlinked (!inode->i_nlink).

This officially enables userspace to push standard, path-backed tmpfs
file descriptors through the LUO session ioctl instead of being
artificially restricted strictly to anonymous memfd pseudofiles.

Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
---
 mm/memfd_luo.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index dcda5bf98646..107115c97bd8 100644
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -9,14 +9,15 @@
  */
 
 /**
- * DOC: Memfd Preservation via LUO
+ * DOC: Memfd and Tmpfs Preservation via LUO
  *
  * Overview
  * ========
  *
- * Memory file descriptors (memfd) can be preserved over a kexec using the Live
- * Update Orchestrator (LUO) file preservation. This allows userspace to
- * transfer its memory contents to the next kernel after a kexec.
+ * Memory file descriptors (memfd) and generic tmpfs files can be preserved
+ * over a kexec using the Live Update Orchestrator (LUO) file preservation.
+ * This allows userspace to transfer its memory contents to the next kernel
+ * after a kexec.
  *
  * The preservation is not intended to be transparent. Only select properties of
  * the file are preserved. All others are reset to default. The preserved
@@ -24,16 +25,17 @@
  *
  * .. note::
  *    The LUO API is not stabilized yet, so the preserved properties of a memfd
- *    are also not stable and are subject to backwards incompatible changes.
+ *    or tmpfs file are also not stable and are subject to backwards
+ *    incompatible changes.
  *
  * .. note::
- *    Currently a memfd backed by Hugetlb is not supported. Memfds created
+ *    Currently a file backed by Hugetlb is not supported. Memfds created
  *    with ``MFD_HUGETLB`` will be rejected.
  *
  * Preserved Properties
  * ====================
  *
- * The following properties of the memfd are preserved across kexec:
+ * The following properties of the memfd/tmpfs file are preserved across kexec:
  *
  * File Contents
  *   All data stored in the file is preserved.
@@ -580,9 +582,7 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
 static bool memfd_luo_can_preserve(struct liveupdate_file_handler *handler,
 				   struct file *file)
 {
-	struct inode *inode = file_inode(file);
-
-	return shmem_file(file) && !inode->i_nlink;
+	return shmem_file(file) || shmem_mapping(file->f_mapping);
 }
 
 static unsigned long memfd_luo_get_id(struct file *file)
-- 
2.55.0.966.g6673acef38-goog