From: John Hubbard <jhubbard@nvidia.com>
To: David Hildenbrand <david@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
<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>,
Linus Torvalds <torvalds@linux-foundation.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>, Jan Kara <jack@suse.cz>,
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
Date: Fri, 15 Jan 2021 19:40:46 -0800 [thread overview]
Message-ID: <cba836c6-4054-c966-8254-a11915351b6b@nvidia.com> (raw)
In-Reply-To: <a21e6fdf-5cac-6fda-242e-7909af96027a@redhat.com>
On 1/15/21 11:46 AM, David Hildenbrand wrote:
>>> 7) There is no easy way to detect if a page really was pinned: we might
>>> have false positives. Further, there is no way to distinguish if it was
>>> pinned with FOLL_WRITE or not (R vs R/W). To perform reliable tracking
>>> we most probably would need more counters, which we cannot fit into
>>> struct page. (AFAIU, for huge pages it's easier).
>>
>> I think this is the real issue. We can only store so much information,
>> so we have to decide which things work and which things are broken. So
>> far someone hasn't presented a way to record everything at least..
>
> I do wonder how many (especially long-term) GUP readers/writers we have
> to expect, and especially, support for a single base page. Do we have a
> rough estimate?
>
> With RDMA, I would assume we only need a single one (e.g., once RDMA
> device; I'm pretty sure I'm wrong, sounds too easy).
> With VFIO I guess we need one for each VFIO container (~ in the worst
> case one for each passthrough device).
> With direct I/O, vmsplice and other GUP users ?? No idea.
>
> If we could somehow put a limit on the #GUP we support, and fail further
> GUP (e.g., -EAGAIN?) once a limit is reached, we could partition the
> refcount into something like (assume max #15 GUP READ and #15 GUP R/W,
> which is most probably a horribly bad choice)
>
> [ GUP READ ][ GUP R/W ] [ ordinary ]
> 31 ... 28 27 ... 24 23 .... 0
>
> But due to saturate handling in "ordinary", we would lose further 2 bits
> (AFAIU), leaving us "only" 22 bits for "ordinary". Now, I have no idea
> how many bits we actually need in practice.
>
> Maybe we need less for GUP READ, because most users want GUP R/W? No idea.
>
> Just wild ideas. Most probably that has already been discussed, and most
> probably people figured that it's impossible :)
>
I proposed this exact idea a few days ago [1]. It's remarkable that we both
picked nearly identical values for the layout! :)
But as the responses show, security problems prevent pursuing that approach.
[1] https://lore.kernel.org/r/45806a5a-65c2-67ce-fc92-dc8c2144d766@nvidia.com
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2021-01-16 3:40 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-10 0:44 [PATCH 0/1] mm: restore full accuracy in COW page reuse Andrea Arcangeli
2021-01-10 0:44 ` [PATCH 1/1] " Andrea Arcangeli
2021-01-10 2:54 ` Andrea Arcangeli
2021-01-11 14:11 ` Kirill A. Shutemov
2021-01-10 0:55 ` [PATCH 0/1] " Linus Torvalds
2021-01-10 1:19 ` Linus Torvalds
2021-01-10 1:37 ` Linus Torvalds
2021-01-10 3:24 ` Andrea Arcangeli
2021-01-10 2:51 ` Andrea Arcangeli
2021-01-10 3:51 ` Linus Torvalds
2021-01-10 19:30 ` Linus Torvalds
2021-01-11 1:18 ` Jason Gunthorpe
2021-01-11 7:26 ` John Hubbard
2021-01-11 12:42 ` Matthew Wilcox
2021-01-11 16:05 ` Jason Gunthorpe
2021-01-11 16:15 ` Michal Hocko
2021-01-11 19:19 ` Linus Torvalds
2021-01-11 22:18 ` Linus Torvalds
2021-01-12 17:07 ` Andy Lutomirski
2021-01-12 23:51 ` Jerome Glisse
2021-01-13 2:16 ` Matthew Wilcox
2021-01-13 2:43 ` Linus Torvalds
2021-01-13 3:31 ` Linus Torvalds
2021-01-13 8:52 ` David Hildenbrand
2021-01-13 8:57 ` David Hildenbrand
2021-01-13 12:32 ` Kirill A. Shutemov
2021-01-13 12:55 ` Matthew Wilcox
2021-01-13 19:54 ` Linus Torvalds
2021-01-13 23:54 ` Peter Xu
2021-01-11 15:52 ` Jason Gunthorpe
2021-01-15 8:59 ` David Hildenbrand
2021-01-15 18:37 ` Jason Gunthorpe
2021-01-15 19:46 ` David Hildenbrand
2021-01-15 19:53 ` Jason Gunthorpe
2021-01-16 3:40 ` John Hubbard [this message]
2021-01-16 11:42 ` David Hildenbrand
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