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Subject: Re: [PATCH] io_uring: fix the bug of child process can't do io task
From:   Jens Axboe <axboe@kernel.dk>
To:     Yinyin Zhu <zhuyinyin@bytedance.com>, viro@zeniv.linux.org.uk
Cc:     linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200915130245.89585-1-zhuyinyin@bytedance.com>
 <e206f1b4-1f22-c3f5-21a6-cec498d9c830@kernel.dk>
Message-ID: <0d66eabc-3b8b-2f84-05b7-981c2b6fe5dd@kernel.dk>
Date:   Tue, 15 Sep 2020 07:36:54 -0600
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On 9/15/20 7:25 AM, Jens Axboe wrote:
> On 9/15/20 7:02 AM, Yinyin Zhu wrote:
>> when parent process setup a io_uring_instance, the ctx->sqo_mm was
>> assigned of parent process'mm. Then it fork a child
>> process. So the child process inherits the io_uring_instance fd from
>> parent process. Then the child process submit a io task to the io_uring
>> instance. The kworker will do the io task actually, and use
>> the ctx->sqo_mm as its mm, but this ctx->sqo_mm is parent process's mm,
>> not the child process's mm. so child do the io task unsuccessfully. To
>> fix this bug, when a process submit a io task to the kworker, assign the
>> ctx->sqo_mm with this process's mm.
> 
> Hmm, what's the test case for this? There's a 5.9 regression where we
> don't always grab the right context for certain linked cases, below
> is the fix. Does that fix your case?

Ah hang on, you're on the 5.4 code base... I think this is a better
approach. Any chance you can test it?

The problem with yours is that you can have multiple pending async
ones, and you can't just re-assign ctx->sqo_mm. That one should only
be used by the SQPOLL thread.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2a539b794f3b..e8a4b4ae7006 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -514,7 +514,7 @@ static inline void io_queue_async_work(struct io_ring_ctx *ctx,
 		}
 	}
 
-	req->task = current;
+	req->task = get_task_struct(current);
 
 	spin_lock_irqsave(&ctx->task_lock, flags);
 	list_add(&req->task_list, &ctx->task_list);
@@ -1832,6 +1832,7 @@ static void io_poll_complete_work(struct work_struct *work)
 	spin_unlock_irq(&ctx->completion_lock);
 
 	io_cqring_ev_posted(ctx);
+	put_task_struct(req->task);
 	io_put_req(req);
 out:
 	revert_creds(old_cred);
@@ -2234,11 +2235,11 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 
 		ret = 0;
 		if (io_req_needs_user(req) && !cur_mm) {
-			if (!mmget_not_zero(ctx->sqo_mm)) {
+			if (!mmget_not_zero(req->task->mm)) {
 				ret = -EFAULT;
 				goto end_req;
 			} else {
-				cur_mm = ctx->sqo_mm;
+				cur_mm = req->task->mm;
 				use_mm(cur_mm);
 				old_fs = get_fs();
 				set_fs(USER_DS);
@@ -2275,6 +2276,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 		}
 
 		/* drop submission reference */
+		put_task_struct(req->task);
 		io_put_req(req);
 
 		if (ret) {

-- 
Jens Axboe