exfat: reduce unnecessary writes during mmap write

During mmap write, exfat_page_mkwrite() currently extends
valid_size to the end of the VMA range. For a large mapping,
this can push valid_size far beyond the page that actually
triggered the fault, resulting in unnecessary writes.

valid_size only needs to extend to the end of the page
being written.

Signed-off-by: Yuling Dong <yuling-dong@qq.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: Yuling Dong

fs/exfat/file.c

@@ -708,21 +708,18 @@ static ssize_t exfat_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 static vm_fault_t exfat_page_mkwrite(struct vm_fault *vmf)
 {
 	int err;
-	struct vm_area_struct *vma = vmf->vma;
-	struct file *file = vma->vm_file;
-	struct inode *inode = file_inode(file);
+	struct inode *inode = file_inode(vmf->vma->vm_file);
 	struct exfat_inode_info *ei = EXFAT_I(inode);
-	loff_t start, end;
+	loff_t new_valid_size;
 
 	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);
+	new_valid_size = ((loff_t)vmf->pgoff + 1) << PAGE_SHIFT;
+	new_valid_size = min(new_valid_size, i_size_read(inode));
 
-	if (ei->valid_size < end) {
-		err = exfat_extend_valid_size(inode, end);
+	if (ei->valid_size < new_valid_size) {
+		err = exfat_extend_valid_size(inode, new_valid_size);
 		if (err < 0) {
 			inode_unlock(inode);
 			return vmf_fs_error(err);