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Subject: Re: [PATCH] io_uring: fix the bug of child process can't do io task
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>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <e206f1b4-1f22-c3f5-21a6-cec498d9c830@kernel.dk>
Date:   Tue, 15 Sep 2020 07:25:30 -0600
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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?


commit 202700e18acbed55970dbb9d4d518ac59b1172c8
Author: Jens Axboe <axboe@kernel.dk>
Date:   Sat Sep 12 13:18:10 2020 -0600

    io_uring: grab any needed state during defer prep
    
    Always grab work environment for deferred links. The assumption that we
    will be running it always from the task in question is false, as exiting
    tasks may mean that we're deferring this one to a thread helper. And at
    that point it's too late to grab the work environment.
    
    Fixes: debb85f496c9 ("io_uring: factor out grab_env() from defer_prep()")
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 175fb647d099..be9d628e7854 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5449,6 +5449,8 @@ static int io_req_defer_prep(struct io_kiocb *req,
 	if (unlikely(ret))
 		return ret;
 
+	io_prep_async_work(req);
+
 	switch (req->opcode) {
 	case IORING_OP_NOP:
 		break;

-- 
Jens Axboe