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    SubjectRe: [PATCH 10/13] PCI: revoke mappings like devmem
    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@ffwll.ch> 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

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