Re: [PATCH 10/13] PCI: revoke mappings like devmem

[Jason Gunthorpe]

On Wed, Oct 07, 2020 at 12:33:06PM -0700, Dan Williams wrote:
> On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> >
> > Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims
> > the region") /dev/kmem zaps ptes when the kernel requests exclusive
> > acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is
> > the default for all driver uses.
> >
> > Except there's two more ways to access pci bars: sysfs and proc mmap
> > support. Let's plug that hole.
> 
> Ooh, yes, lets.
> 
> >
> > For revoke_devmem() to work we need to link our vma into the same
> > address_space, with consistent vma->vm_pgoff. ->pgoff is already
> > adjusted, because that's how (io_)remap_pfn_range works, but for the
> > mapping we need to adjust vma->vm_file->f_mapping. Usually that's done
> > at ->open time, but that's a bit tricky here with all the entry points
> > and arch code. So instead create a fake file and adjust vma->vm_file.
> 
> I don't think you want to share the devmem inode for this, this should
> be based off the sysfs inode which I believe there is already only one
> instance per resource. In contrast /dev/mem can have multiple inodes
> because anyone can just mknod a new character device file, the same
> problem does not exist for sysfs.

The inode does not come from the filesystem char/mem.c creates a
singular anon inode in devmem_init_inode()

Seems OK to use this more widely, but it feels a bit weird to live in
char/memory.c.

This is what got me thinking maybe this needs to be a bit bigger
generic infrastructure - eg enter this scheme from fops mmap and
everything else is in mm/user_iomem.c

Jason