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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 14:17:50 -0800
Message-ID: <CAHk-=wjhpTa616YdTfincMV1LqqRWqokKtd1ARdMOPd1Wra8Pg@mail.gmail.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, 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>,
        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>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
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On Thu, Jan 7, 2021 at 2:03 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> If you could stop mentioning UFFDIO_WRITEPROTECT and only focus on
> softdirty/clear_refs, maybe you wouldn't think my judgment is biased
> towards clear_refs/softdirty too.

So I think we can agree that even that softdirty case we can just
handle by "don't do that then".

if a page is pinned, the dirty bit of it makes no sense, because it
might be dirtied complately asynchronously by the pinner.

So I think none of the softdirty stuff should touch pinned pages. I
think it falls solidly under that "don't do it then".

Just skipping over them in clear_soft_dirty[_pmd]() doesn't look that
hard, does it?

            Linus