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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 14:24:59 -0800
Message-ID: <CAHk-=wgDhSzz56=us=X2PXHh2XfvcYc76PkarZYNnVaUG=iSmw@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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On Thu, Jan 7, 2021 at 2:20 PM Linus Torvalds
<torvalds@linux-foundation.org> 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