Re: [PATCH 2/3] userfaultfd: UFFDIO_REMAP uABI

[Nadav Amit]

> On Sep 14, 2023, at 8:26 AM, Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> 
> + 	if (!pte_same(ptep_clear_flush(src_vma, src_addr, src_pte),
> +       	orig_src_pte))
> + 		BUG_ON(1);

Just a minor detail regarding these few lines:

Besides the less-than-ideal use of BUG_ON() here, I think that this code
assumes that the PTE cannot change at this point. However, as the PTE was
still mapped at this point, I think the access and dirty bits can be set.

tl;dr: this appears to be triggerable by userspace.

[ as for the performance of this code, the lack of batching would mean
  that for multithreaded applications where more than a single page is
  remapped, performance would suffer ]