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>

diff --git a/fs/file.c b/fs/file.c
index 0a4f3bdb2dec6284a0c7b9687213137f2eecb250..51ddcff0081a65038404b668d8813970cbc1f3b7 100644 (file)
--- 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);