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Date:   Thu, 24 Sep 2020 15:39:53 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Peter Xu <peterx@redhat.com>
Cc:     John Hubbard <jhubbard@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Michal Hocko <mhocko@suse.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
Message-ID: <20200924183953.GG9916@ziepe.ca>
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On Thu, Sep 24, 2020 at 02:34:18PM -0400, Peter Xu wrote:

> > >        // RDONLY gup
> > >        pin_user_pages(buf, !WRITE);
> > >        // pte of buf duplicated on both sides
> > >        fork();
> > >        mprotect(buf, WRITE);
> > >        *buf = 1;
> > >        // buf page replaced as cow triggered
> > > 
> > > Currently when fork() we'll happily share a pinned read-only page with the
> > > child by copying the pte directly.  
> > 
> > Why? This series prevents that, the page will be maybe_dma_pinned, so
> > fork() will copy it.
> 
> With the extra mprotect(!WRITE), I think we'll see a !pte_write() entry.  Then
> it'll not go into maybe_dma_pinned() at all since cow==false.

Hum that seems like a problem in this patch, we still need to do the
DMA pinned logic even if the pte is already write protected.

Jason