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

[zhangfei]

Hi, Dave

On 2020/1/16 äå12:43, Dave Jiang wrote:
> 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.

Thanks Dave for the advice.
Will look into it, may need some time to investigate.
I would like to make an additional patch for this issue, since it does 
not impact the main function.

Thanks