fs/file: optimize close_range() complexity from O(N) to O(Sparse)

In close_range(), the kernel traditionally performs a linear scan over the
[fd, max_fd] range, resulting in O(N) complexity where N is the range size.
For processes with sparse FD tables, this is inefficient as it checks many
unallocated slots.

This patch optimizes __range_close() by using find_next_bit() on the
open_fds bitmap to skip holes. This shifts the algorithmic complexity from
O(Range Size) to O(Active FDs), providing a significant performance boost
for large-range close operations on sparse file descriptor tables.

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
Link: https://patch.msgid.link/20260123081221.659125-1-realwujing@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: realwujing

fs/file.c

@@ -777,23 +777,29 @@ static inline void __range_close(struct files_struct *files, unsigned int fd,
 				 unsigned int max_fd)
 {
 	struct file *file;
+	struct fdtable *fdt;
 	unsigned n;
 
 	spin_lock(&files->file_lock);
-	n = last_fd(files_fdtable(files));
+	fdt = files_fdtable(files);
+	n = last_fd(fdt);
 	max_fd = min(max_fd, n);
 
-	for (; fd <= max_fd; fd++) {
+	for (fd = find_next_bit(fdt->open_fds, max_fd + 1, fd);
+	     fd <= max_fd;
+	     fd = find_next_bit(fdt->open_fds, max_fd + 1, fd + 1)) {
 		file = file_close_fd_locked(files, fd);
 		if (file) {
 			spin_unlock(&files->file_lock);
 			filp_close(file, files);
 			cond_resched();
 			spin_lock(&files->file_lock);
+			fdt = files_fdtable(files);
 		} else if (need_resched()) {
 			spin_unlock(&files->file_lock);
 			cond_resched();
 			spin_lock(&files->file_lock);
+			fdt = files_fdtable(files);
 		}
 	}
 	spin_unlock(&files->file_lock);