Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <B1B85771-B211-4FCC-AEEF-BDFD37332C25@vmware.com>
 <20210107200402.31095-1-aarcange@redhat.com> <20210107200402.31095-3-aarcange@redhat.com>
 <CAHk-=whg-91=EF=8=ayyDQGx_3iuWKp3aHUkDCDkgUb15Yh8AQ@mail.gmail.com>
 <X/d2DyLfXZmBIreY@redhat.com> <CAHk-=wjs9v-hp_7HV_TrTmisu7pNX=MwZ62ZV82i0evLhPwS1Q@mail.gmail.com>
 <4100a6f5-ab0b-f7e5-962f-ea1dbcb1e47e@nvidia.com>
In-Reply-To: <4100a6f5-ab0b-f7e5-962f-ea1dbcb1e47e@nvidia.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 14:00:35 -0800
Message-ID: <CAHk-=wjde11Wz+GiVeuttdAPaNBrNydkvUcVm3xBmVWjwA-kNQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        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@linux.intel.com>,
        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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> >
> > Now, I do agree that from a QoI standpoint, it would be really lovely
> > if we actually enforced it. I'm not entirely sure we can, but maybe it
> > would be reasonable to use that
> >
> >    mm->has_pinned && page_maybe_dma_pinned(page)
> >
> > at least as the beginning of a heuristic.
> >
> > In fact, I do think that "page_maybe_dma_pinned()" could possibly be
> > made stronger than it is. Because at *THAT* point, we might say "we
>
> What exactly did you have in mind, to make it stronger? I think the
> answer is in this email but I don't quite see it yet...

Literally just adding a " && page_mapcount(page) == 1" in there
(probably best done inside page_maybe_dma_pinned() itself)

> Direct IO pins, on the other hand, are more transient. We can probably live
> without tagging Direct IO pages as FOLL_PIN. I think.

Yes. I think direct-IO writes should be able to just do a transient
GUP, and if it causes a COW fault that isn't coherent, that's the
correct semantics, I think (ie the direct-IO will see the original
data, the COW faulter will get it's own private copy to make changes
to).

I think pinning should be primarily limited to things that _require_
coherency (ie you pin because you're going to do some active two-way
communication using that page)

Does that match your thinking?

               Linus