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Date:   Wed, 13 Jan 2021 15:32:32 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        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>,
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        Kees Cook <keescook@chromium.org>,
        Leon Romanovsky <leonro@nvidia.com>,
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        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
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On Tue, Jan 12, 2021 at 07:31:07PM -0800, Linus Torvalds wrote:
> On Tue, Jan 12, 2021 at 6:16 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > The thing about the speculative page cache references is that they can
> > temporarily bump a refcount on a page which _used_ to be in the page
> > cache and has now been reallocated as some other kind of page.
> 
> Oh, and thinking about this made me think we might actually have a
> serious bug here, and it has nothing what-so-ever to do with COW, GUP,
> or even the page count itself.
> 
> It's unlikely enough that I think it's mostly theoretical, but tell me
> I'm wrong.
> 
> PLEASE tell me I'm wrong:
> 
> CPU1 does page_cache_get_speculative under RCU lock
> 
> CPU2 frees and re-uses the page
> 
>     CPU1                CPU2
>     ----                ----
> 
>     page = xas_load(&xas);
>     if (!page_cache_get_speculative(page))
>             goto repeat;
>     .. succeeds ..
> 
>                         remove page from XA
>                         release page
>                         reuse for something else

How can it be reused if CPU1 hold reference to it?

> 
>     .. and then re-check ..
>     if (unlikely(page != xas_reload(&xas))) {
>             put_page(page);
>             goto repeat;
>     }
> 

-- 
 Kirill A. Shutemov