exfat: move extend valid_size into ->page_mkwrite()

It is not a good way to extend valid_size to the end of the
mmap area by writing zeros in mmap. Because after calling mmap,
no data may be written, or only a small amount of data may be
written to the head of the mmap area.

This commit moves extending valid_size to exfat_page_mkwrite().
In exfat_page_mkwrite() only extend valid_size to the starting
position of new data writing, which reduces unnecessary writing
of zeros.

If the block is not mapped and is marked as new after being
mapped for writing, block_write_begin() will zero the page
cache corresponding to the block, so there is no need to call
zero_user_segment() in exfat_file_zeroed_range(). And after moving
extending valid_size to exfat_page_mkwrite(), the data written by
mmap will be copied to the page cache but the page cache may be
not mapped to the disk. Calling zero_user_segment() will cause
the data written by mmap to be cleared. So this commit removes
calling zero_user_segment() from exfat_file_zeroed_range() and
renames exfat_file_zeroed_range() to exfat_extend_valid_size().

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: YuezhangMo

fs/exfat/file.c

@@ -532,32 +532,32 @@ int exfat_file_fsync(struct file *filp, loff_t start, loff_t end, int datasync)
 	return blkdev_issue_flush(inode->i_sb->s_bdev);
 }
 
-static int exfat_file_zeroed_range(struct file *file, loff_t start, loff_t end)
+static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 {
 	int err;
+	loff_t pos;
 	struct inode *inode = file_inode(file);
+	struct exfat_inode_info *ei = EXFAT_I(inode);
 	struct address_space *mapping = inode->i_mapping;
 	const struct address_space_operations *ops = mapping->a_ops;
 
-	while (start < end) {
-		u32 zerofrom, len;
+	pos = ei->valid_size;
+	while (pos < new_valid_size) {
+		u32 len;
 		struct folio *folio;
 
-		zerofrom = start & (PAGE_SIZE - 1);
-		len = PAGE_SIZE - zerofrom;
-		if (start + len > end)
-			len = end - start;
+		len = PAGE_SIZE - (pos & (PAGE_SIZE - 1));
+		if (pos + len > new_valid_size)
+			len = new_valid_size - pos;
 
-		err = ops->write_begin(file, mapping, start, len, &folio, NULL);
+		err = ops->write_begin(file, mapping, pos, len, &folio, NULL);
 		if (err)
 			goto out;
 
-		folio_zero_range(folio, offset_in_folio(folio, start), len);
-
-		err = ops->write_end(file, mapping, start, len, len, folio, NULL);
+		err = ops->write_end(file, mapping, pos, len, len, folio, NULL);
 		if (err < 0)
 			goto out;
-		start += len;
+		pos += len;
 
 		balance_dirty_pages_ratelimited(mapping);
 		cond_resched();
@@ -585,7 +585,7 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		goto unlock;
 
 	if (pos > valid_size) {
-		ret = exfat_file_zeroed_range(file, valid_size, pos);
+		ret = exfat_extend_valid_size(file, pos);
 		if (ret < 0 && ret != -ENOSPC) {
 			exfat_err(inode->i_sb,
 				"write: fail to zero from %llu to %llu(%zd)",
@@ -619,26 +619,46 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	return ret;
 }
 
-static int exfat_file_mmap(struct file *file, struct vm_area_struct *vma)
+static vm_fault_t exfat_page_mkwrite(struct vm_fault *vmf)
 {
-	int ret;
+	int err;
+	struct vm_area_struct *vma = vmf->vma;
+	struct file *file = vma->vm_file;
 	struct inode *inode = file_inode(file);
 	struct exfat_inode_info *ei = EXFAT_I(inode);
-	loff_t start = ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
-	loff_t end = min_t(loff_t, i_size_read(inode),
+	loff_t start, end;
+
+	if (!inode_trylock(inode))
+		return VM_FAULT_RETRY;
+
+	start = ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
+	end = min_t(loff_t, i_size_read(inode),
 			start + vma->vm_end - vma->vm_start);
 
-	if ((vma->vm_flags & VM_WRITE) && ei->valid_size < end) {
-		ret = exfat_file_zeroed_range(file, ei->valid_size, end);
-		if (ret < 0) {
-			exfat_err(inode->i_sb,
-				  "mmap: fail to zero from %llu to %llu(%d)",
-				  start, end, ret);
-			return ret;
+	if (ei->valid_size < end) {
+		err = exfat_extend_valid_size(file, end);
+		if (err < 0) {
+			inode_unlock(inode);
+			return vmf_fs_error(err);
 		}
 	}
 
-	return generic_file_mmap(file, vma);
+	inode_unlock(inode);
+
+	return filemap_page_mkwrite(vmf);
+}
+
+static const struct vm_operations_struct exfat_file_vm_ops = {
+	.fault		= filemap_fault,
+	.map_pages	= filemap_map_pages,
+	.page_mkwrite	= exfat_page_mkwrite,
+};
+
+static int exfat_file_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	file_accessed(file);
+	vma->vm_ops = &exfat_file_vm_ops;
+	return 0;
 }
 
 const struct file_operations exfat_file_operations = {