[PATCH? v2] fput: don't abuse task_work_add() too much

[Oleg Nesterov <oleg@redhat.com>]

On 09/07, Oleg Nesterov wrote:
>
> Oh, I disagree. But I guess I can't convince you/Eric/Linus, so I have
> to shut up.
>
>
> Damn. But I can't relax ;) Al, Linus, could you comment the patch below?
>
> Not for inclusion, lacks the changelog/testing, fput() can be simplified.
> But as you can see it is simple. With this patch task_work_add(____fput)
> will be called only once by (say) do_exit() path. ->fput_list does not
> need any serialization / atomic ops / etc. Probably we also need to move
> cond_resched() from task_work_run() to ____fput() after this patch.
>
> Again, it is not that I think this actually makes sense, but since you
> dislike these 275ms...
>
> What do you think?

Yes, task_struct->fput_list is ugly. We can avoid it, but then we need
another ->next pointer in struct file. Perhaps we can reuse ->f_version?

This way the change looks really simple and not too bad to me. Although
I am not sure you will agree.

Oleg.
---

diff --git a/fs/file_table.c b/fs/file_table.c
index 294174d..c34b666 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -241,7 +241,15 @@ static void delayed_fput(struct work_struct *unused)
 
 static void ____fput(struct callback_head *work)
 {
-	__fput(container_of(work, struct file, f_u.fu_rcuhead));
+	struct file *file = container_of(work, struct file, f_u.fu_rcuhead);
+	struct file *next;
+
+	do {
+		next = file->f_next_put;
+		__fput(file);
+		file = next;
+
+	} while (file);
 }
 
 /*
@@ -267,9 +275,21 @@ void fput(struct file *file)
 		struct task_struct *task = current;
 
 		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
+			struct callback_head *work = READ_ONCE(task->task_works);
+			struct file *prev;
+
+			if (work && work->func == ____fput) {
+				prev = container_of(work, struct file, f_u.fu_rcuhead);
+				file->f_next_put = prev->f_next_put;
+				prev->f_next_put = file;
+				return;
+			}
+
 			init_task_work(&file->f_u.fu_rcuhead, ____fput);
-			if (!task_work_add(task, &file->f_u.fu_rcuhead, true))
+			if (!task_work_add(task, &file->f_u.fu_rcuhead, true)) {
+				file->f_next_put = NULL;
 				return;
+			}
 			/*
 			 * After this task has run exit_task_work(),
 			 * task_work_add() will fail.  Fall through to delayed
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0774487..9381527 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -849,7 +849,10 @@ struct file {
 	const struct cred	*f_cred;
 	struct file_ra_state	f_ra;
 
-	u64			f_version;
+	union {
+		u64		f_version;
+		struct file	*f_next_put;
+	};
 #ifdef CONFIG_SECURITY
 	void			*f_security;
 #endif