[PATCH v2 4/9] mm/mshare: Add a read operation for msharefs files

[Khalid Aziz <khalid.aziz@oracle.com>]

When a new file is created under msharefs, allocate a new mm_struct
that will hold the VMAs for mshare region. Also allocate structure
to defines the mshare region and add a read operation to the file
that returns this information about the mshare region. Currently
this information is returned as a struct:

struct mshare_info {
	unsigned long start;
	unsigned long size;
};

This gives the start address for mshare region and its size.

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
---
 include/uapi/linux/mman.h |  5 +++
 mm/mshare.c               | 64 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index f55bc680b5b0..56fe446e24b1 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -41,4 +41,9 @@
 #define MAP_HUGE_2GB	HUGETLB_FLAG_ENCODE_2GB
 #define MAP_HUGE_16GB	HUGETLB_FLAG_ENCODE_16GB
 
+struct mshare_info {
+	unsigned long start;
+	unsigned long size;
+};
+
 #endif /* _UAPI_LINUX_MMAN_H */
diff --git a/mm/mshare.c b/mm/mshare.c
index 2d5924d39221..d238b68b0576 100644
--- a/mm/mshare.c
+++ b/mm/mshare.c
@@ -22,8 +22,14 @@
 #include <uapi/linux/magic.h>
 #include <uapi/linux/limits.h>
 #include <uapi/linux/mman.h>
+#include <linux/sched/mm.h>
 
 static struct super_block *msharefs_sb;
+struct mshare_data {
+	struct mm_struct *mm;
+	refcount_t refcnt;
+	struct mshare_info *minfo;
+};
 
 static const struct inode_operations msharefs_dir_inode_ops;
 static const struct inode_operations msharefs_file_inode_ops;
@@ -34,8 +40,29 @@ msharefs_open(struct inode *inode, struct file *file)
 	return simple_open(inode, file);
 }
 
+static ssize_t
+msharefs_read(struct kiocb *iocb, struct iov_iter *iov)
+{
+	struct mshare_data *info = iocb->ki_filp->private_data;
+	size_t ret;
+	struct mshare_info m_info;
+
+	if (info->minfo != NULL) {
+		m_info.start = info->minfo->start;
+		m_info.size = info->minfo->size;
+	} else {
+		m_info.start = 0;
+		m_info.size = 0;
+	}
+	ret = copy_to_iter(&m_info, sizeof(m_info), iov);
+	if (!ret)
+		return -EFAULT;
+	return ret;
+}
+
 static const struct file_operations msharefs_file_operations = {
 	.open		= msharefs_open,
+	.read_iter	= msharefs_read,
 	.llseek		= no_llseek,
 };
 
@@ -73,12 +100,43 @@ static struct dentry
 	return ERR_PTR(-ENOMEM);
 }
 
+static int
+msharefs_fill_mm(struct inode *inode)
+{
+	struct mm_struct *mm;
+	struct mshare_data *info = NULL;
+	int retval = 0;
+
+	mm = mm_alloc();
+	if (!mm) {
+		retval = -ENOMEM;
+		goto err_free;
+	}
+
+	info = kzalloc(sizeof(*info), GFP_KERNEL);
+	if (!info) {
+		retval = -ENOMEM;
+		goto err_free;
+	}
+	info->mm = mm;
+	info->minfo = NULL;
+	refcount_set(&info->refcnt, 1);
+	inode->i_private = info;
+
+	return 0;
+
+err_free:
+	if (mm)
+		mmput(mm);
+	kfree(info);
+	return retval;
+}
+
 static struct inode
 *msharefs_get_inode(struct super_block *sb, const struct inode *dir,
 			umode_t mode)
 {
 	struct inode *inode = new_inode(sb);
-
 	if (inode) {
 		inode->i_ino = get_next_ino();
 		inode_init_owner(&init_user_ns, inode, dir, mode);
@@ -89,6 +147,10 @@ static struct inode
 		case S_IFREG:
 			inode->i_op = &msharefs_file_inode_ops;
 			inode->i_fop = &msharefs_file_operations;
+			if (msharefs_fill_mm(inode) != 0) {
+				discard_new_inode(inode);
+				inode = ERR_PTR(-ENOMEM);
+			}
 			break;
 		case S_IFDIR:
 			inode->i_op = &msharefs_dir_inode_ops;
-- 
2.32.0