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Date:   Tue, 22 Sep 2020 08:54:36 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jann Horn <jannh@google.com>
Cc:     Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <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>,
        John Hubbard <jhubbard@nvidia.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
Message-ID: <20200922115436.GG8409@ziepe.ca>
References: <20200921211744.24758-1-peterx@redhat.com>
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 <CAG48ez25krKvd5hWqn0R3w5_AAPVWtWKofiOHEfKaB2+YoDoPw@mail.gmail.com>
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On Tue, Sep 22, 2020 at 12:47:11AM +0200, Jann Horn wrote:
> On Tue, Sep 22, 2020 at 12:30 AM Peter Xu <peterx@redhat.com> wrote:
> > On Mon, Sep 21, 2020 at 11:43:38PM +0200, Jann Horn wrote:
> > > On Mon, Sep 21, 2020 at 11:17 PM Peter Xu <peterx@redhat.com> wrote:
> > > > (Commit message collected from Jason Gunthorpe)
> > > >
> > > > Reduce the chance of false positive from page_maybe_dma_pinned() by keeping
> > > > track if the mm_struct has ever been used with pin_user_pages(). mm_structs
> > > > that have never been passed to pin_user_pages() cannot have a positive
> > > > page_maybe_dma_pinned() by definition.
> > >
> > > There are some caveats here, right? E.g. this isn't necessarily true
> > > for pagecache pages, I think?
> >
> > Sorry I didn't follow here.  Could you help explain with some details?
> 
> The commit message says "mm_structs that have never been passed to
> pin_user_pages() cannot have a positive page_maybe_dma_pinned() by
> definition"; but that is not true for pages which may also be mapped
> in a second mm and may have been passed to pin_user_pages() through
> that second mm (meaning they must be writable over there and not
> shared with us via CoW).

The message does need a few more words to explain this trick can only
be used with COW'able pages.
 
> Process A:
> 
> fd_a = open("/foo/bar", O_RDWR);
> mapping_a = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, fd_a, 0);
> pin_user_pages(mapping_a, 1, ...);
> 
> Process B:
> 
> fd_b = open("/foo/bar", O_RDONLY);
> mapping_b = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd_b, 0);
> *(volatile char *)mapping_b;
> 
> At this point, process B has never called pin_user_pages(), but
> page_maybe_dma_pinned() on the page at mapping_b would return true.

My expectation is the pin_user_pages() should have already broken the
COW for the MAP_PRIVATE, so process B should not have a
page_maybe_dma_pinned()

Jason