Received-SPF: pass (google.com: best guess record for domain of linux-crypto-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v11 2/4] uacce: add uacce driver
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        jonathan.cameron@huawei.com, dave.jiang@intel.com,
        grant.likely@arm.com, jean-philippe <jean-philippe@linaro.org>,
        Jerome Glisse <jglisse@redhat.com>,
        ilias.apalodimas@linaro.org, francois.ozog@linaro.org,
        kenneth-lee-2012@foxmail.com, Wangzhou <wangzhou1@hisilicon.com>,
        "haojian . zhuang" <haojian.zhuang@linaro.org>,
        guodong.xu@linaro.org, linux-accelerators@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        iommu@lists.linux-foundation.org,
        Kenneth Lee <liguozhu@hisilicon.com>,
        Zaibo Xu <xuzaibo@huawei.com>
References: <1578710919-12141-1-git-send-email-zhangfei.gao@linaro.org>
 <1578710919-12141-3-git-send-email-zhangfei.gao@linaro.org>
 <20200111194006.GD435222@kroah.com>
 <053ccd05-4f11-5be6-47c2-eee5c2f1fdc4@linaro.org>
 <20200114145934.GA1960403@kroah.com>
 <c71b402c-a185-50a7-2827-c1836cc6c237@linaro.org>
 <20200115120212.GA3270387@kroah.com>
From:   zhangfei <zhangfei.gao@linaro.org>
Message-ID: <750bb4b6-20a9-96b0-5801-5b8bff8cc3b5@linaro.org>
Date:   Wed, 15 Jan 2020 22:11:56 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.0
MIME-Version: 1.0
In-Reply-To: <20200115120212.GA3270387@kroah.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-crypto-owner@vger.kernel.org
Precedence: bulk


On 2020/1/15 下午8:02, Greg Kroah-Hartman wrote:
> On Wed, Jan 15, 2020 at 07:18:34PM +0800, zhangfei wrote:
>> Hi, Greg
>>
>> On 2020/1/14 下午10:59, Greg Kroah-Hartman wrote:
>>> On Mon, Jan 13, 2020 at 11:34:55AM +0800, zhangfei wrote:
>>>> Hi, Greg
>>>>
>>>> Thanks for the review.
>>>>
>>>> On 2020/1/12 上午3:40, Greg Kroah-Hartman wrote:
>>>>> On Sat, Jan 11, 2020 at 10:48:37AM +0800, Zhangfei Gao wrote:
>>>>>> +static int uacce_fops_open(struct inode *inode, struct file *filep)
>>>>>> +{
>>>>>> +	struct uacce_mm *uacce_mm = NULL;
>>>>>> +	struct uacce_device *uacce;
>>>>>> +	struct uacce_queue *q;
>>>>>> +	int ret = 0;
>>>>>> +
>>>>>> +	uacce = xa_load(&uacce_xa, iminor(inode));
>>>>>> +	if (!uacce)
>>>>>> +		return -ENODEV;
>>>>>> +
>>>>>> +	if (!try_module_get(uacce->parent->driver->owner))
>>>>>> +		return -ENODEV;
>>>>> Why are you trying to grab the module reference of the parent device?
>>>>> Why is that needed and what is that going to help with here?
>>>>>
>>>>> This shouldn't be needed as the module reference of the owner of the
>>>>> fileops for this module is incremented, and the "parent" module depends
>>>>> on this module, so how could it be unloaded without this code being
>>>>> unloaded?
>>>>>
>>>>> Yes, if you build this code into the kernel and the "parent" driver is a
>>>>> module, then you will not have a reference, but when you remove that
>>>>> parent driver the device will be removed as it has to be unregistered
>>>>> before that parent driver can be removed from the system, right?
>>>>>
>>>>> Or what am I missing here?
>>>> The refcount here is preventing rmmod "parent" module after fd is opened,
>>>> since user driver has mmap kernel memory to user space, like mmio, which may
>>>> still in-use.
>>>>
>>>> With the refcount protection, rmmod "parent" module will fail until
>>>> application free the fd.
>>>> log like: rmmod: ERROR: Module hisi_zip is in use
>>> But if the "parent" module is to be unloaded, it has to unregister the
>>> "child" device and that will call the destructor in here and then you
>>> will tear everything down and all should be good.
>>>
>>> There's no need to "forbid" a module from being unloaded, even if it is
>>> being used.  Look at all networking drivers, they work that way, right?
>> Thanks Greg for the kind suggestion.
>>
>> I still have one uncertainty.
>> Does uacce has to block process continue accessing the mmapped area when
>> remove "parent" module?
>> Uacce can block device access the physical memory when parent module call
>> uacce_remove.
>> But application is still running, and suppose it is not the kernel driver's
>> responsibility to call unmap.
>>
>> I am looking for some examples in kernel,
>> looks vfio does not block process continue accessing when
>> vfio_unregister_iommu_driver either.
>>
>> In my test, application will keep waiting after rmmod parent, until ctrl+c,
>> when unmap is called.
>> During the process, kernel does not report any error.
>>
>> Do you have any advice?
> Is there no way for the kernel to invalidate the memory and tell the
> process to stop?  tty drivers do this for when they are removed from the
> system.
>
> Anyway, this is all very rare, no kernel module is ever unloaded on a
> real system, that is only for when developers are working on them, so
> it's probably not that big of an issue, right?
>
Thanks Greg, will update a new version while ignoring this first.

Thanks