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Date:   Wed, 13 Jan 2021 02:16:19 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
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        Will Deacon <will@kernel.org>,
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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 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.