From: Andy Lutomirski <luto@kernel.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>,
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>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
Date: Tue, 12 Jan 2021 09:07:31 -0800 [thread overview]
Message-ID: <CALCETrWEAOj28Y6SaBVuxahuRmk9exp1fqcJO0ibGKbFw4HQ4A@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wje9r3fREBdZcOu=NihGczBtkqkhXRPDhY-ZkNVv=thiQ@mail.gmail.com>
On Mon, Jan 11, 2021 at 2:18 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, Jan 11, 2021 at 11:19 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> Actually, what I think might be a better model is to actually
> strengthen the rules even more, and get rid of GUP_PIN_COUNTING_BIAS
> entirely.
>
> What we could do is just make a few clear rules explicit (most of
> which we already basically hold to). Starting from that basic
>
> (a) Anonymous pages are made writable (ie COW) only when they have a
> page_count() of 1
Seems reasonable to me.
>
> That very simple rule then automatically results in the corollary
>
> (b) a writable page in a COW mapping always starts out reachable
> _only_ from the page tables
Seems reasonable. I guess that if the COW is triggered by GUP, then
it starts out reachable only from the page tables but then because
reachable through GUP very soon thereafter.
>
> and now we could have a couple of really simple new rules:
>
> (c) we never ever make a writable page in a COW mapping read-only
> _unless_ it has a page_count() of 1
I don't love this. Having mprotect() fail in a multithreaded process
because another thread happens to be doing a short-lived IO seems like
it may result in annoying intermittent bugs.
As I understand it, the issue is that the way we determine that we
need to COW a COWable page is that we see that it's read-only. It
would be nice if we could separately track "the VMA allows writes" and
"this PTE points to a page that is private to the owning VMA", but
maybe there's no bit available for the latter other than looking at RO
vs RW directly.
>
> (d) we never create a swap cache page out of a writable COW mapping page
>
> Now, if you combine these rules, the whole need for the
> GUP_PIN_COUNTING_BIAS basically goes away.
>
> Why? Because we know that the _only_ thing that can elevate the
> refcount of a writable COW page is GUP - we'll just make sure nothing
> else touches it.
How common is !FOLL_WRITE GUP? We could potentially say that a
short-term !FOLL_WRITE GUP is permitted on an RO COW page and that a
subsequent COW on the page will wait for the GUP to go away. This
might be too big a can of worms for the benefit it would provide,
though.
--Andy
next prev parent reply other threads:[~2021-01-12 17:07 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 [this message]
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
2021-01-16 11:42 ` David Hildenbrand
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