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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 11 Jan 2021 11:19:33 -0800
Message-ID: <CAHk-=wipa-9wEuWHBjourmXAVHdeqDa59UxW6ZJ_Oqg6-Dwvdw@mail.gmail.com>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     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>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        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>
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On Sun, Jan 10, 2021 at 11:27 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> There is at least one way to improve this part of it--maybe.

It's problematic..

> 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.

It may be unlikely under real loads.

But we've actually had overflow issues on this because rather than
real loads you can do attack loads (ie "lots of processes, lots of
pipe file descriptors, lots of vmsplice() operations on the same
page".

We had to literally add that conditional "try_get_page()" that
protects against overflow..

           Linus