In prep for the following support for page cache sharing based mmap,
allocate an anonymous file of corresponding blob, so that we can link
associated vma to the blob later.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c  | 74 +++++++++++++++++++++++++++++++++++++++++++++
 fs/erofs/internal.h |  1 +
 2 files changed, 75 insertions(+)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 81f96c910277..f715b3efcc77 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -4,6 +4,8 @@
  * Copyright (C) 2022, Bytedance Inc. All rights reserved.
  */
 #include <linux/fscache.h>
+#include <linux/file.h>
+#include <linux/anon_inodes.h>
 #include "internal.h"
 
 static DEFINE_MUTEX(erofs_domain_list_lock);
@@ -22,6 +24,11 @@ struct erofs_fscache_request {
 	refcount_t		ref;
 };
 
+struct erofs_fscache_share_file_info {
+	erofs_off_t pa;
+	pgoff_t max_idx;
+};
+
 static struct erofs_fscache_request *erofs_fscache_req_alloc(struct address_space *mapping,
 					     loff_t start, size_t len)
 {
@@ -316,6 +323,73 @@ const struct address_space_operations erofs_fscache_access_aops = {
 	.readahead = erofs_fscache_readahead,
 };
 
+static int erofs_fscache_share_meta_release(struct inode *inode, struct file *filp)
+{
+	kfree(filp->private_data);
+	filp->private_data = NULL;
+	return 0;
+}
+
+static const struct file_operations erofs_fscache_share_meta_fops = {
+	.release	= erofs_fscache_share_meta_release,
+};
+
+static int erofs_fscache_share_file_release(struct inode *inode, struct file *filp)
+{
+	fput(filp->private_data);
+	filp->private_data = NULL;
+	return 0;
+}
+
+static int erofs_fscache_share_file_open(struct inode *inode, struct file *filp)
+{
+	/* since page cache sharing is enabled only when i_size <= chunk_size */
+	struct erofs_map_blocks map = {}; /* .m_la = 0 */
+	struct erofs_map_dev mdev;
+	struct inode *realinode;
+	struct file *realfile;
+	struct erofs_fscache_share_file_info *finfo;
+	int ret;
+
+	ret = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
+	if (ret)
+		return ret;
+
+	mdev = (struct erofs_map_dev) {
+		.m_deviceid = map.m_deviceid,
+		.m_pa = map.m_pa,
+	};
+	ret = erofs_map_dev(inode->i_sb, &mdev);
+	if (ret)
+		return ret;
+
+	finfo = kzalloc(sizeof(struct erofs_fscache_share_file_info), GFP_KERNEL);
+	if (!finfo)
+		return -ENOMEM;
+	finfo->pa = mdev.m_pa;
+	finfo->max_idx = DIV_ROUND_UP(mdev.m_pa + inode->i_size, PAGE_SIZE);
+
+	realinode = mdev.m_fscache->inode;
+	ihold(realinode);
+	realfile = alloc_file_pseudo(realinode, filp->f_path.mnt, "[erofs]",
+				     O_RDONLY, &erofs_fscache_share_meta_fops);
+	if (IS_ERR(realfile)) {
+		iput(realinode);
+		kfree(finfo);
+		return PTR_ERR(realfile);
+	}
+
+	realfile->private_data = finfo;
+	filp->private_data = realfile;
+	return 0;
+}
+
+const struct file_operations erofs_fscache_share_file_fops = {
+	.llseek		= generic_file_llseek,
+	.open		= erofs_fscache_share_file_open,
+	.release	= erofs_fscache_share_file_release,
+};
+
 static void erofs_fscache_domain_put(struct erofs_domain *domain)
 {
 	mutex_lock(&erofs_domain_list_lock);
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index b3d04bc2d279..7c6a7a2d9acf 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -616,6 +616,7 @@ struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
 void erofs_fscache_unregister_cookie(struct erofs_fscache *fscache);
 
 extern const struct address_space_operations erofs_fscache_access_aops;
+extern const struct file_operations erofs_fscache_share_file_fops;
 #else
 static inline int erofs_fscache_register_fs(struct super_block *sb)
 {
-- 
2.19.1.6.gb485710b