Re: [PATCH v3 1/2] io_uring: avoid whole io_wq_work copy for requests completed inline

[Pavel Begunkov <asml.silence@gmail.com>]

On 28/05/2020 12:15, Xiaoguang Wang wrote:
> If requests can be submitted and completed inline, we don't need to
> initialize whole io_wq_work in io_init_req(), which is an expensive
> operation, add a new 'REQ_F_WORK_INITIALIZED' to control whether
> io_wq_work is initialized.

It looks nicer. Especially if you'd add a helper as Jens supposed.

The other thing, even though I hate treating a part of the fields differently
from others, I don't like ->creds tossing either.

Did you consider trying using only ->work.creds without adding req->creds? like
in the untested incremental below. init_io_work() there is misleading, should be
somehow played around better.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4dd3295d74f6..4086561ce444 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -643,7 +643,6 @@ struct io_kiocb {
 	unsigned int		flags;
 	refcount_t		refs;
 	struct task_struct	*task;
-	const struct cred	*creds;
 	unsigned long		fsize;
 	u64			user_data;
 	u32			result;
@@ -894,8 +893,16 @@ static const struct file_operations io_uring_fops;
 static inline void init_io_work(struct io_kiocb *req,
 			void (*func)(struct io_wq_work **))
 {
-	req->work = (struct io_wq_work){ .func = func };
-	req->flags |= REQ_F_WORK_INITIALIZED;
+	struct io_wq_work *work = &req->work;
+
+	/* work->creds are already initialised by a user */
+	work->list.next = NULL;
+	work->func = func;
+	work->files = NULL;
+	work->mm = NULL;
+	work->fs = NULL;
+	work->flags = REQ_F_WORK_INITIALIZED;
+	work->task_pid = 0;
 }
 struct sock *io_uring_get_socket(struct file *file)
 {
@@ -1019,15 +1026,9 @@ static inline void io_req_work_grab_env(struct io_kiocb *req,
 		mmgrab(current->mm);
 		req->work.mm = current->mm;
 	}
+	if (!req->work.creds)
+		req->work.creds = get_current_cred();

-	if (!req->work.creds) {
-		if (!req->creds)
-			req->work.creds = get_current_cred();
-		else {
-			req->work.creds = req->creds;
-			req->creds = NULL;
-		}
-	}
 	if (!req->work.fs && def->needs_fs) {
 		spin_lock(&current->fs->lock);
 		if (!current->fs->in_exec) {
@@ -1044,6 +1045,12 @@ static inline void io_req_work_grab_env(struct io_kiocb *req,

 static inline void io_req_work_drop_env(struct io_kiocb *req)
 {
+	/* always init'ed, put before REQ_F_WORK_INITIALIZED check */
+	if (req->work.creds) {
+		put_cred(req->work.creds);
+		req->work.creds = NULL;
+	}
+
 	if (!(req->flags & REQ_F_WORK_INITIALIZED))
 		return;

@@ -1051,10 +1058,6 @@ static inline void io_req_work_drop_env(struct io_kiocb *req)
 		mmdrop(req->work.mm);
 		req->work.mm = NULL;
 	}
-	if (req->work.creds) {
-		put_cred(req->work.creds);
-		req->work.creds = NULL;
-	}
 	if (req->work.fs) {
 		struct fs_struct *fs = req->work.fs;

@@ -5901,12 +5904,12 @@ static int io_init_req(struct io_ring_ctx *ctx, struct
io_kiocb *req,

 	id = READ_ONCE(sqe->personality);
 	if (id) {
-		req->creds = idr_find(&ctx->personality_idr, id);
-		if (unlikely(!req->creds))
+		req->work.creds = idr_find(&ctx->personality_idr, id);
+		if (unlikely(!req->work.creds))
 			return -EINVAL;
-		get_cred(req->creds);
+		get_cred(req->work.creds);
 	} else
-		req->creds = NULL;
+		req->work.creds = NULL;

 	/* same numerical values with corresponding REQ_F_*, safe to copy */
 	req->flags |= sqe_flags;

-- 
Pavel Begunkov