fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()

alloc_fd() has a sanity check inside to make sure the struct file mapping to the
allocated fd is NULL. Remove this sanity check since it can be assured by
exisitng zero initilization and NULL set when recycling fd. Meanwhile, add
likely/unlikely and expand_file() call avoidance to reduce the work under
file_lock.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Yu Ma <yu.ma@intel.com>
Link: https://lore.kernel.org/r/20240717145018.3972922-2-yu.ma@intel.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: intelmy

fs/file.c

@@ -496,27 +496,29 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 	if (fd < files->next_fd)
 		fd = files->next_fd;
 
-	if (fd < fdt->max_fds)
+	if (likely(fd < fdt->max_fds))
 		fd = find_next_fd(fdt, fd);
 
 	/*
 	 * N.B. For clone tasks sharing a files structure, this test
 	 * will limit the total number of files that can be opened.
 	 */
 	error = -EMFILE;
-	if (fd >= end)
+	if (unlikely(fd >= end))
 		goto out;
 
-	error = expand_files(files, fd);
-	if (error < 0)
-		goto out;
+	if (unlikely(fd >= fdt->max_fds)) {
+		error = expand_files(files, fd);
+		if (error < 0)
+			goto out;
 
-	/*
-	 * If we needed to expand the fs array we
-	 * might have blocked - try again.
-	 */
-	if (error)
-		goto repeat;
+		/*
+		 * If we needed to expand the fs array we
+		 * might have blocked - try again.
+		 */
+		if (error)
+			goto repeat;
+	}
 
 	if (start <= files->next_fd)
 		files->next_fd = fd + 1;
@@ -527,13 +529,6 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 	else
 		__clear_close_on_exec(fd, fdt);
 	error = fd;
-#if 1
-	/* Sanity check */
-	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
-		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
-		rcu_assign_pointer(fdt->fd[fd], NULL);
-	}
-#endif
 
 out:
 	spin_unlock(&files->file_lock);
@@ -599,7 +594,7 @@ void fd_install(unsigned int fd, struct file *file)
 		rcu_read_unlock_sched();
 		spin_lock(&files->file_lock);
 		fdt = files_fdtable(files);
-		BUG_ON(fdt->fd[fd] != NULL);
+		WARN_ON(fdt->fd[fd] != NULL);
 		rcu_assign_pointer(fdt->fd[fd], file);
 		spin_unlock(&files->file_lock);
 		return;