io_uring: track mm through current->mm

As a preparation for extracting request init bits, remove self-coded mm
tracking from io_submit_sqes(), but rely on current->mm. It's more
convenient, than passing this piece of state in other functions.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5812,8 +5812,7 @@ static void io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 }
 
 static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
-			  struct file *ring_file, int ring_fd,
-			  struct mm_struct **mm, bool async)
+			  struct file *ring_file, int ring_fd, bool async)
 {
 	struct io_submit_state state, *statep = NULL;
 	struct io_kiocb *link = NULL;
@@ -5870,13 +5869,12 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 			break;
 		}
 
-		if (io_op_defs[req->opcode].needs_mm && !*mm) {
+		if (io_op_defs[req->opcode].needs_mm && !current->mm) {
 			if (unlikely(!mmget_not_zero(ctx->sqo_mm))) {
 				err = -EFAULT;
 				goto fail_req;
 			}
 			use_mm(ctx->sqo_mm);
-			*mm = ctx->sqo_mm;
 		}
 
 		req->needs_fixed_file = async;
@@ -5902,10 +5900,19 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 	return submitted;
 }
 
+static inline void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
+{
+	struct mm_struct *mm = current->mm;
+
+	if (mm) {
+		unuse_mm(mm);
+		mmput(mm);
+	}
+}
+
 static int io_sq_thread(void *data)
 {
 	struct io_ring_ctx *ctx = data;
-	struct mm_struct *cur_mm = NULL;
 	const struct cred *old_cred;
 	mm_segment_t old_fs;
 	DEFINE_WAIT(wait);
@@ -5946,11 +5953,7 @@ static int io_sq_thread(void *data)
 			 * adding ourselves to the waitqueue, as the unuse/drop
 			 * may sleep.
 			 */
-			if (cur_mm) {
-				unuse_mm(cur_mm);
-				mmput(cur_mm);
-				cur_mm = NULL;
-			}
+			io_sq_thread_drop_mm(ctx);
 
 			/*
 			 * We're polling. If we're within the defined idle
@@ -6014,7 +6017,7 @@ static int io_sq_thread(void *data)
 		}
 
 		mutex_lock(&ctx->uring_lock);
-		ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
+		ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
 		mutex_unlock(&ctx->uring_lock);
 		timeout = jiffies + ctx->sq_thread_idle;
 	}
@@ -6023,10 +6026,7 @@ static int io_sq_thread(void *data)
 		task_work_run();
 
 	set_fs(old_fs);
-	if (cur_mm) {
-		unuse_mm(cur_mm);
-		mmput(cur_mm);
-	}
+	io_sq_thread_drop_mm(ctx);
 	revert_creds(old_cred);
 
 	kthread_parkme();
@@ -7507,13 +7507,8 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			wake_up(&ctx->sqo_wait);
 		submitted = to_submit;
 	} else if (to_submit) {
-		struct mm_struct *cur_mm;
-
 		mutex_lock(&ctx->uring_lock);
-		/* already have mm, so io_submit_sqes() won't try to grab it */
-		cur_mm = ctx->sqo_mm;
-		submitted = io_submit_sqes(ctx, to_submit, f.file, fd,
-					   &cur_mm, false);
+		submitted = io_submit_sqes(ctx, to_submit, f.file, fd, false);
 		mutex_unlock(&ctx->uring_lock);
 
 		if (submitted != to_submit)