linux-kernel - Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page

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Date:   Wed, 2 Mar 2022 16:59:34 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when
 R/O-pinning a possibly shared anonymous page

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.

Jason