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Date:   Mon, 11 Jan 2021 12:42:20 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
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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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        Hugh Dickins <hughd@google.com>,
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        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 Sun, Jan 10, 2021 at 11:26:57PM -0800, John Hubbard wrote:
> IMHO, a lot of the bits in page _refcount are still being wasted (even
> after GUP_PIN_COUNTING_BIAS overloading), because it's unlikely that
> there are many callers of gup/pup per page. If anyone points out that
> that is wrong, then the rest of this falls apart, but...if we were to
> make a rule that "only a very few FOLL_GET or FOLL_PIN pins are ever
> simultaneously allowed on a given page", then several things become
> possible:

There's "the normal case" and then there's "the attacker case" where
someone's deliberately trying to wrap page->_refcount.  There are lots of
interesting games people can play with an anon page, like stuffing it into
(lots of) pipes, forking lots of children, starting lots of O_DIRECT I/O
against it to a FUSE filesystem that's deliberately engineered to be slow.

We have some protection against that, but I'm not 100% sure it's working,
and making it easier to increase refcount in large chunks makes it more
likely that we would defeat that protection.