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

[Linus Torvalds]

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