linux-kernel - Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice instead of char dev

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Date:   Tue, 15 Mar 2022 05:22:15 +0200
From:   Mika Penttilä <mpenttil@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        apopple@...dia.com, jhubbard@...dia.com, rcampbell@...dia.com,
        vbabka@...e.cz
Subject: Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice
 instead of char dev

Hi Jason and thanks for your comments..

On 14.3.2022 20.24, Jason Gunthorpe wrote:
> On Fri, Mar 11, 2022 at 05:30:50AM +0200, mpenttil@...hat.com wrote:
>> From: Mika Penttilä <mpenttil@...hat.com>
>>
>> HMM selftests use an in-kernel pseudo device to emulate device private
>> memory. The pseudo device registers a major device range for two pseudo
>> device instances. User space has a script that reads /proc/devices in
>> order to find the assigned major number, and sends that to mknod(1),
>> once for each node.
>>
>> This duplicates a fair amount of boilerplate that misc device can do
>> instead.
>>
>> Change this to use misc device, which makes the device node names appear
>> for us. This also enables udev-like processing if desired.
> 
> This is borderline the wrong way to use misc devices, they should
> never be embedded into other structs like this. It works out here
> because they are eventually only placed in a static array, but still
> it is a generally bad pattern to see.

Could you elaborate on this one? We have many in-tree usages of the same 
pattern, like:

drivers/video/fbdev/pxa3xx-gcu.c
drivers/platform/goldfish/goldfish_pipe.c
drivers/virt/vboxguest/vboxguest_linux.c
drivers/virt/nitro_enclaves/ne_pci_dev.c
drivers/watchdog/cpwd.c
drivers/platform/x86/toshiba_acpi.c
drivers/platform/x86/wmi.c
drivers/platform/surface/surface_dtx.c
drivers/bus/fsl-mc/fsl-mc-uapi.c
drivers/misc/dw-xdata-pcie.c
drivers/input/serio/serio_raw.c
fs/dlm/user.c
lib/test_kmod.c

You mention "placed in a static array", are you seeing a potential 
lifetime issue or what? Many of the examples above embed miscdevice in a 
dynamically allocated object also.

The file object's private_data holds a pointer to the miscdevice, and 
fops_get() pins the module. So freeing the objects miscdevice is 
embedded in at module_exit time should be fine. But, as you said, in 
this case the miscdevices are statically allocated, so that shouldn't be 
an issue either. But maybe I'm missing something?

> 
>> Delete the /proc/devices parsing from the user-space test script, now
>> that it is unnecessary.
> 
> This is all because the cdev is being used wrong - it get all this
> stuff it should be done via cdev_device_add() and a dmirror_device
> needs a struct device to hold the sysfs.
> 

I think using cdev_add ends up in the same results in device_* api 
sense. miscdevice acting like a mux at a higher abstraction level 
simplifies the code.


> Jason
> 


Thanks,

Mika