mm/filemap: introduce generic_file_*_mmap_prepare() helpers

Since commit c84bf6d ("mm: introduce new .mmap_prepare() file
callback"), the f_op->mmap() hook has been deprecated in favour of
f_op->mmap_prepare().

The generic mmap handlers are very simple, so we can very easily convert
these in advance of converting file systems which use them.

This patch does so.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/30622c1f0b98c66840bc8c02668bda276a810b70.1750099179.git.lorenzo.stoakes@oracle.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: ljskernel

include/linux/fs.h

@@ -3393,8 +3393,10 @@ extern void inode_add_lru(struct inode *inode);
 extern int sb_set_blocksize(struct super_block *, int);
 extern int sb_min_blocksize(struct super_block *, int);
 
-extern int generic_file_mmap(struct file *, struct vm_area_struct *);
-extern int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
+int generic_file_mmap(struct file *, struct vm_area_struct *);
+int generic_file_mmap_prepare(struct vm_area_desc *desc);
+int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
+int generic_file_readonly_mmap_prepare(struct vm_area_desc *desc);
 extern ssize_t generic_write_checks(struct kiocb *, struct iov_iter *);
 int generic_write_checks_count(struct kiocb *iocb, loff_t *count);
 extern int generic_write_check_limits(struct file *file, loff_t pos,

mm/filemap.c

@@ -3814,6 +3814,18 @@ int generic_file_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+int generic_file_mmap_prepare(struct vm_area_desc *desc)
+{
+	struct file *file = desc->file;
+	struct address_space *mapping = file->f_mapping;
+
+	if (!mapping->a_ops->read_folio)
+		return -ENOEXEC;
+	file_accessed(file);
+	desc->vm_ops = &generic_file_vm_ops;
+	return 0;
+}
+
 /*
  * This is for filesystems which do not implement ->writepage.
  */
@@ -3823,6 +3835,13 @@ int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 	return generic_file_mmap(file, vma);
 }
+
+int generic_file_readonly_mmap_prepare(struct vm_area_desc *desc)
+{
+	if (is_shared_maywrite(desc->vm_flags))
+		return -EINVAL;
+	return generic_file_mmap_prepare(desc);
+}
 #else
 vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf)
 {
@@ -3832,15 +3851,25 @@ int generic_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	return -ENOSYS;
 }
+int generic_file_mmap_prepare(struct vm_area_desc *desc)
+{
+	return -ENOSYS;
+}
 int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	return -ENOSYS;
 }
+int generic_file_readonly_mmap_prepare(struct vm_area_desc *desc)
+{
+	return -ENOSYS;
+}
 #endif /* CONFIG_MMU */
 
 EXPORT_SYMBOL(filemap_page_mkwrite);
 EXPORT_SYMBOL(generic_file_mmap);
+EXPORT_SYMBOL(generic_file_mmap_prepare);
 EXPORT_SYMBOL(generic_file_readonly_mmap);
+EXPORT_SYMBOL(generic_file_readonly_mmap_prepare);
 
 static struct folio *do_read_cache_folio(struct address_space *mapping,
 		pgoff_t index, filler_t filler, struct file *file, gfp_t gfp)