> Looking at this code some more.. How is it even correct?
>
> 1633 if (!isolate_lru_page(head)) {
> 1634 list_add_tail(&head->lru, &cma_page_list);
>
> Here we are only running under the read side of the mmap sem so multiple
> GUPs can be calling that sequence in parallel. I don't see any
> obvious exclusion that will prevent corruption of head->lru. The first
> GUP thread to do isolate_lru_page() will ClearPageLRU() and the second
> GUP thread will be a NOP for isolate_lru_page().
>
> They will both race list_add_tail and other list ops. That is not OK.
Good question. I studied it, and I do not see how this is OK. Worse,
this race is also exposable as a syscall instead of via driver: two
move_pages() run simultaneously. Perhaps in other places?
move_pages()
kernel_move_pages()
mmget()
do_pages_move()
add_page_for_migratio()
mmap_read_lock(mm);
list_add_tail(&head->lru, pagelist); <- Not protected
>
> > What I meant is the users of the interface do it incrementally not in
> > large chunks. For example:
> >
> > vfio_pin_pages_remote
> > vaddr_get_pfn
> > ret = pin_user_pages_remote(mm, vaddr, 1, flags |
> > FOLL_LONGTERM, page, NULL, NULL);
> > 1 -> pin only one pages at a time
>
> I don't know why vfio does this, it is why it so ridiculously slow at
> least.
Agreed.
>
> Jason
