linux-kernel - Re: [PATCH] io_uring: fix deferred req iovec leak

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Message-ID: <28cacc0c-68e4-a9d1-bb5e-03dbeff8a586@kernel.dk>
Date:   Thu, 6 Feb 2020 13:16:28 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring: fix deferred req iovec leak

On 2/6/20 1:00 PM, Pavel Begunkov wrote:
> On 06/02/2020 22:56, Jens Axboe wrote:
>> On 2/6/20 10:16 AM, Pavel Begunkov wrote:
>>> On 06/02/2020 20:04, Pavel Begunkov wrote:
>>>> On 06/02/2020 19:51, Pavel Begunkov wrote:
>>>>> After defer, a request will be prepared, that includes allocating iovec
>>>>> if needed, and then submitted through io_wq_submit_work() but not custom
>>>>> handler (e.g. io_rw_async()/io_sendrecv_async()). However, it'll leak
>>>>> iovec, as it's in io-wq and the code goes as follows:
>>>>>
>>>>> io_read() {
>>>>> 	if (!io_wq_current_is_worker())
>>>>> 		kfree(iovec);
>>>>> }
>>>>>
>>>>> Put all deallocation logic in io_{read,write,send,recv}(), which will
>>>>> leave the memory, if going async with -EAGAIN.
>>>>>
>>>> Interestingly, this will fail badly if it returns -EAGAIN from io-wq context.
>>>> Apparently, I need to do v2.
>>>>
>>> Or not...
>>> Jens, can you please explain what's with the -EAGAIN handling in
>>> io_wq_submit_work()? Checking the code, it seems neither of
>>> read/write/recv/send can return -EAGAIN from async context (i.e.
>>> force_nonblock=false). Are there other ops that can do it?
>>
>> Nobody should return -EAGAIN with force_nonblock=false, they should
>> end the io_kiocb inline for that.
>>
> 
> If so for those 4, then the patch should work well.

Maybe I'm dense, but I'm not seeing the leak? We have two cases here:

- The number of vecs is less than UIO_FASTIOV, in which case we use the
  on-stack inline_vecs. If we need to go async, we copy that inline vec
  to our async_ctx area.

- The number of vecs is more than UIO_FASTIOV, this is where iovec is
  allocated by the vec import. If we make it to completion here, we
  free it at the end of eg io_read(). If we need to go async, we stash
  that pointer in our async_ctx area and free it when the work item
  has run and completed.

-- 
Jens Axboe