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Date:   Mon, 11 Jan 2021 17:11:47 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        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>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.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>,
        Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse
Message-ID: <20210111141147.wzk5rvycsiitnnrt@box>
References: <20210110004435.26382-1-aarcange@redhat.com>
 <20210110004435.26382-2-aarcange@redhat.com>
 <X/psTXZp9xNgX3gE@redhat.com>
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On Sat, Jan 09, 2021 at 09:54:05PM -0500, Andrea Arcangeli wrote:
> Hello,
> 
> On Sat, Jan 09, 2021 at 07:44:35PM -0500, Andrea Arcangeli wrote:
> > allowing a child to corrupt memory in the parent. That's a problem
> > that could happen not-maliciously too. So the scenario described
> 
> I updated the above partly quoted sentence since in the previous
> version it didn't have full accuracy:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=fc5a76b1c14e5e6cdc64ece306fc03773662d98a
> 
> "However since a single transient GUP pin on a tail page, would elevate
> the page_count for all other tail pages (unlike the mapcount which is
> subpage granular), the COW page reuse inaccuracy would then cross
> different vmas and the effect would happen at a distance in vma of
> different processes. A single GUP pin taken on a subpage mapped in a
> different process could trigger 511 false positive COWs copies in the
> local process, after a fork()."
> 
> This a best effort to try to document all side effects, but it'd be
> great to hear from Kirill too on the above detail to have
> confirmation.

Yes, this side effect is possible. But I wouldn't worry about too much. If
it routinely happens in a real workloads (I doubt it does), the workload
can tune it with MADV_NOHUGEPAGE/MADV_DONTFORK/MADV_WIPEONFORK or
something.

-- 
 Kirill A. Shutemov