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Date:   Thu, 24 Sep 2020 14:34:18 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
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: <20200924183418.GJ79898@xz-x1>
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 <20200924143517.GD79898@xz-x1>
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On Thu, Sep 24, 2020 at 03:15:01PM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 24, 2020 at 01:55:31PM -0400, Peter Xu wrote:
> > On Thu, Sep 24, 2020 at 01:51:52PM -0300, Jason Gunthorpe wrote:
> > > > Regarding the solution here, I think we can also cover read-only fast-gup too
> > > > in the future - IIUC what we need to do is to make it pte_protnone() instead of
> > > > pte_wrprotect(), then in the fault handler we should identify this special
> > > > pte_protnone() against numa balancing (change_prot_numa()).  I think it should
> > > > work fine too, iiuc, because I don't think we should migrate a page at all if
> > > > it's pinned for any reason...
> > 
> > [1]
> > 
> > > 
> > > With your COW breaking patch the read only fast-gup should break the
> > > COW because of the write protect, just like for the write side. Not
> > > seeing why we need to do something more?
> > 
> > Consider this sequence of a parent process managed to fork() a child:
> > 
> >        buf = malloc();

Sorry! I think I missed something like:

           mprotect(buf, !WRITE);

Here.

> >        // 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.

> 
> > As a summary: imho the important thing is we should not allow any kind of
> > sharing of any dma page, even it's pinned for read.
> 
> Any sharing that results in COW. MAP_SHARED is fine, for instance

Oh right, MAP_SHARED is definitely special.

Thanks,

-- 
Peter Xu