[PATCH v4 12/14] mm: add shmem_zero_setup_desc()

Lorenzo Stoakes
2025-09-17T19:12:36Z

Add the ability to set up a shared anonymous mapping based on a VMA
descriptor rather than a VMA.

This is a prerequisite for converting to the char mm driver to use the
mmap_prepare hook.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
include/linux/shmem_fs.h | 3 ++-
mm/shmem.c | 41 ++++++++++++++++++++++++++++++++--------
2 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 0e47465ef0fd..5b368f9549d6 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -94,7 +94,8 @@ extern struct file *shmem_kernel_file_setup(const char *name, loff_t size,
unsigned long flags);
extern struct file *shmem_file_setup_with_mnt(struct vfsmount *mnt,
const char *name, loff_t size, unsigned long flags);
-extern int shmem_zero_setup(struct vm_area_struct *);
+int shmem_zero_setup(struct vm_area_struct *vma);
+int shmem_zero_setup_desc(struct vm_area_desc *desc);
extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
unsigned long len, unsigned long pgoff, unsigned long flags);
extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);
diff --git a/mm/shmem.c b/mm/shmem.c
index df02a2e0ebbb..72aa176023de 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -5893,14 +5893,9 @@ struct file *shmem_file_setup_with_mnt(struct vfsmount *mnt, const char *name,
}
EXPORT_SYMBOL_GPL(shmem_file_setup_with_mnt);

-/**
- * shmem_zero_setup - setup a shared anonymous mapping
- * @vma: the vma to be mmapped is prepared by do_mmap
- */
-int shmem_zero_setup(struct vm_area_struct *vma)
+static struct file *__shmem_zero_setup(unsigned long start, unsigned long end, vm_flags_t vm_flags)
{
- struct file *file;
- loff_t size = vma->vm_end - vma->vm_start;
+ loff_t size = end - start;

/*
* Cloning a new file under mmap_lock leads to a lock ordering conflict
@@ -5908,7 +5903,18 @@ int shmem_zero_setup(struct vm_area_struct *vma)
* accessible to the user through its mapping, use S_PRIVATE flag to
* bypass file security, in the same way as shmem_kernel_file_setup().
*/
- file = shmem_kernel_file_setup("dev/zero", size, vma->vm_flags);
+ return shmem_kernel_file_setup("dev/zero", size, vm_flags);
+}
+
+/**
+ * shmem_zero_setup - setup a shared anonymous mapping
+ * @vma: the vma to be mmapped is prepared by do_mmap
+ * Returns: 0 on success, or error
+ */
+int shmem_zero_setup(struct vm_area_struct *vma)
+{
+ struct file *file = __shmem_zero_setup(vma->vm_start, vma->vm_end, vma->vm_flags);
+
if (IS_ERR(file))
return PTR_ERR(file);

@@ -5920,6 +5926,25 @@ int shmem_zero_setup(struct vm_area_struct *vma)
return 0;
}

+/**
+ * shmem_zero_setup_desc - same as shmem_zero_setup, but determined by VMA
+ * descriptor for convenience.
+ * @desc: Describes VMA
+ * Returns: 0 on success, or error
+ */
+int shmem_zero_setup_desc(struct vm_area_desc *desc)
+{
+ struct file *file = __shmem_zero_setup(desc->start, desc->end, desc->vm_flags);
+
+ if (IS_ERR(file))
+ return PTR_ERR(file);
+
+ desc->vm_file = file;
+ desc->vm_ops = &shmem_anon_vm_ops;
+
+ return 0;
+}
+
/**
* shmem_read_folio_gfp - read into page cache, using specified page allocation flags.
* @mapping: the folio's address_space
--
2.51.0