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

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Date:   Wed, 13 Jan 2021 02:16:19 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        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 Mon, Jan 11, 2021 at 02:18:13PM -0800, Linus Torvalds wrote:
> The whole "optimistic page references throigh page cache" etc are
> complete non-issues, because the whole point is that we already know
> it's not a page cache page. There is simply no other way to reach that
> page than through GUP.

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.

Now, this is obviously rare, so if it's only a performance question,
it'll be fine.  If there's a correctness issue with copying pages that
would otherwise not have been copied, then it's a problem.