linux-kernel - Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush

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Message-ID: <CAHk-=wgDhSzz56=us=X2PXHh2XfvcYc76PkarZYNnVaUG=iSmw@mail.gmail.com>
Date:   Thu, 7 Jan 2021 14:24:59 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

On Thu, Jan 7, 2021 at 2:20 PM Linus Torvalds
<torvalds@...ux-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