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