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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 12:25:52 -0800
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Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     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>,
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        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>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
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On Thu, Jan 7, 2021 at 12:17 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I still think the real fix is "Don't do that then", and just take the
> write lock.

The alternative, of course, is to just make sure the page table flush
is done inside the page table lock (and then we make COW do the copy
inside of it).

But this whole "we know UFFD breaks all rules, we'll add even more
crap to it" approach is horrendous.

               Linus