Re: [PATCH v11 2/4] uacce: add uacce driver

[Dave Jiang]

On 1/15/20 4:18 AM, zhangfei wrote:
> Hi, Greg
>
> On 2020/1/14 äå10:59, 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 äå3:40, 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)
> > > > > +{
> > > > > +ÂÂÂ struct uacce_mm *uacce_mm = NULL;
> > > > > +ÂÂÂ struct uacce_device *uacce;
> > > > > +ÂÂÂ struct uacce_queue *q;
> > > > > +ÂÂÂ int ret = 0;
> > > > > +
> > > > > +ÂÂÂ uacce = xa_load(&uacce_xa, iminor(inode));
> > > > > +ÂÂÂ if (!uacce)
> > > > > +ÂÂÂÂÂÂÂ return -ENODEV;
> > > > > +
> > > > > +ÂÂÂ if (!try_module_get(uacce->parent->driver->owner))
> > > > > +ÂÂÂÂÂÂÂ 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?

Would it work to call unmap_mapping_range() on the char dev 
inode->i_mappings? I think you need to set the vma->fault function ptr 
for the vm_operations_struct in the original mmap(). After the mappings 
are unmapped, you can set a state variable to trigger the return of 
VM_FAULT_SIGBUS in the ->fault function when the user app accesses the 
mmap region again and triggers a page fault. The user app needs to be 
programmed to catch exceptions to deal with that.

> > > > > +static void uacce_release(struct device *dev)
> > > > > +{
> > > > > +ÂÂÂ struct uacce_device *uacce = to_uacce_device(dev);
> > > > > +
> > > > > +ÂÂÂ kfree(uacce);
> > > > > +ÂÂÂ uacce = NULL;
> > > > That line didn't do anything :)
> > > Yes, this is a mistake.
> > > It is up to caller to set to NULL to prevent release multi times.
> > Release function is called by the driver core which will not touch the
> > value again.
> Yes, I understand, it's my mistake. Will remove it.
>
> Thanks