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

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Date:   Thu, 7 Jan 2021 14:20:27 -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 2:14 PM John Hubbard <jhubbard@...dia.com> wrote:
> >
> > Literally just adding a " && page_mapcount(page) == 1" in there
> > (probably best done inside page_maybe_dma_pinned() itself)
>
> Well, that means that pages that are used for pinned DMA like this, can
> not be shared with other processes. Is that an acceptable limitation
> for the RDMA users? It seems a bit constraining, at first glance anyway.

Hmm, add a check for the page being PageAnon(), perhaps?

If it's a shared vma, then the page can be pinned shared with multiple
mappings, I agree.

So yeah, I didn't think it through entirely.. And maybe I'm still
missing something else..

          Linus