linux-kernel - Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush

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Date:   Thu, 7 Jan 2021 14:00:35 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@...dia.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