linux-kernel - Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

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Date:   Wed, 13 Jan 2021 15:32:32 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Nadav Amit <nadav.amit@...il.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

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@...radead.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