Re: [PATCH v3 12/16] PCI: Obey iomem restrictions for procfs mmap

[Jason Gunthorpe]

On Wed, Oct 21, 2020 at 04:42:11PM +0200, Daniel Vetter wrote:

> Uh yes. In drivers/gpu this isn't a problem because we only install
> ptes from the vm_ops->fault handler. So no races. And I don't think
> you can fix this otherwise through holding locks: mmap_sem we can't
> hold because before vma_link we don't even know which mm_struct is
> involved, so can't solve the race. Plus this would be worse that
> mm_take_all_locks used by mmu notifier. And the address_space
> i_mmap_lock is also no good since it's not held during the ->mmap
> callback, when we write the ptes. And the resource locks is even less
> useful, since we're not going to hold that at vma_link() time for
> sure.
> 
> Hence delaying the pte writes after the vma_link, which means ->fault
> time, looks like the only way to close this gap.

> Trouble is I have no idea how to do this cleanly ...

How about add a vm_ops callback 'install_pages'/'prefault_pages' ?

Call it after vm_link() - basically just move the remap_pfn, under
some other lock, into there.

Jason