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Date:   Thu, 3 Mar 2022 09:07:18 +0100
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Subject: Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when
 R/O-pinning a possibly shared anonymous page
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To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Liang Zhang <zhangliang5@huawei.com>,
        Pedro Gomes <pedrodemargomes@gmail.com>,
        Oded Gabbay <oded.gabbay@gmail.com>, linux-mm@kvack.org
References: <20220224122614.94921-1-david@redhat.com>
 <20220224122614.94921-13-david@redhat.com>
 <20220302165559.GU219866@nvidia.com>
 <fb895ba3-9d7e-7421-d5c6-f5e7a2d1231a@redhat.com>
 <20220302205934.GV219866@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20220302205934.GV219866@nvidia.com>
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On 02.03.22 21:59, Jason Gunthorpe wrote:
> On Wed, Mar 02, 2022 at 09:38:09PM +0100, David Hildenbrand wrote:
> 
>> (a) I want a R/O pin to observe file modifications.
>> (b) I want the R/O pin to *not* observe file modifications but observe
>>     my (eventual? if any) private modifications,
> 
> A scenario I know that motivated this is fairly straightfoward:
> 
>    static char data[] = {};
> 
>    ibv_reg_mr(data, READ_ONLY)
>    data[0] = 1
>    .. go touch data via DMA ..
> 
> We want to reliably observe the '1'
> 
> What is happening under the covers is that 'data' is placed in the
> .data segment and becomes a file backed MAP_PRIVATE page. The write
> COWs that page
> 
> It would work OK if it was in .bss instead
> 
> I think the FOLL_FORCE is there because otherwise the trick doesn't
> work on true RO pages and the API becomes broken.

Thanks for the nice example, it matches what John brought up in respect
to MAP_PRIVATE semantics.

-- 
Thanks,

David / dhildenb