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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Dec 2021 19:38:39 -0800
Message-ID: <CAHk-=wj7eSOhbWDeADL_BJKLzdDF5s_5R9v7d-4P3L6v1T3mpQ@mail.gmail.com>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
To:     Nadav Amit <namit@vmware.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        David Hildenbrand <david@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        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>,
        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>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
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On Fri, Dec 17, 2021 at 7:30 PM Nadav Amit <namit@vmware.com> wrote:
>
> In such a case, I do think it makes sense to fail uffd-wp (when
> page_count() > 1), and in a prototype I am working on I do something
> like that.

Ack. If uddf-wp finds a page that is pinned, just skip it as not
write-protectable.

Because some of the pinners might be writing to it, of course - just
not through the page tables.

So that sounds like the right thing to do. I _think_ we discussed this
the last time this came up. I have some dim memory of that. Jason,
ring a bell?

             Linus