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Date:   Wed, 13 Jan 2021 12:55:32 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        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>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        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 0/1] mm: restore full accuracy in COW page reuse
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On Wed, Jan 13, 2021 at 03:32:32PM +0300, Kirill A. Shutemov wrote:
> 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?

Yes, Linus mis-stated it:

    page = xas_load(&xas);
                        remove page from XA
                        release page
                        reuse for something else
    if (!page_cache_get_speculative(page))
            goto repeat;
    if (unlikely(page != xas_reload(&xas))) {
            put_page(page);

... but as David pointed out, I fixed this in e320d3012d25