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

[Suren Baghdasaryan]

On Thu, Sep 14, 2023 at 2:57 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
>
> > 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.

At this point we are holding PTLs for both PTEs (see
double_pt_lock()).  Can a PTE be modified from under us in this
situation?

>
> 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 ]

Thanks for the note! I'll see if it's possible to implement some
batching mechanism here.
Thanks,
Suren.