Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

[John Hubbard]

On 1/7/21 2:24 PM, Linus Torvalds wrote:
> On Thu, Jan 7, 2021 at 2:20 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > Hmm, add a check for the page being PageAnon(), perhaps?
> >
> > If it's a shared vma, then the page can be pinned shared with multiple
> > mappings, I agree.
>
> Or check the vma directly for whether it's a COW vma. That's probably
> a more obvious test, but would have to be done outside of
> page_maybe_dma_pinned().
>
> For example, in copy_present_page(), we've already done that COW-vma
> test, so if we want to strengthen just _that_ test, then it would be
> sufficient to just add a
>
>          /* This cannot be a pinned page if it has more than one mapping */
>          if (page_mappings(page) != 1)
>                  return 1;
>
> to copy_present_page() along with the existing page_maybe_dma_pinned() test.
>
> No?
>
>              Linus

That approach makes me a lot happier, yes. Because it doesn't add constraints
to the RDMA cases, but still does what we need here.


thanks,
--
John Hubbard
NVIDIA