[PATCH v2] fs/file: optimize close_range() complexity from O(N) to O(Sparse)

[Qiliang Yuan <realwujing@gmail.com>]

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>
---
v2:
  - Recalculate fdt after re-acquiring file_lock to avoid UAF if the
    table is expanded/reallocated during filp_close() or cond_resched().
v1:
  - Initial optimization using find_next_bit() on open_fds bitmap to
    skip holes, improving complexity to O(Active FDs).

 fs/file.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 0a4f3bdb2dec..51ddcff0081a 100644
--- a/fs/file.c
+++ b/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);
-- 
2.51.0