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Date:   Wed, 2 Dec 2020 14:37:33 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Andrea Arcangeli <aarcange@redhat.com>
cc:     Peter Xu <peterx@redhat.com>, Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions
 on reads
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References: <20201130230603.46187-1-peterx@redhat.com> <20201201125927.GB11935@casper.infradead.org> <20201201223033.GG3277@xz-x1> <X8bZk3jTGU8QyJWc@redhat.com>
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On Tue, 1 Dec 2020, Andrea Arcangeli wrote:
> 
> Any suggestions on how to have the per-vaddr per-mm _PAGE_UFFD_WP bit
> survive the pte invalidates in a way that remains associated to a
> certain vaddr in a single mm (so it can shoot itself in the foot if it
> wants, but it can't interfere with all other mm sharing the shmem
> file) would be welcome...

I think it has to be a new variety of swap-like non_swap_entry() pte,
see include/linux/swapops.h.  Anything else would be more troublesome.

Search for non_swap_entry and for migration_entry, to find places that 
might need to learn about this new variety.

IIUC you only need a single value, no need to carve out another whole
swp_type: could probably be swp_offset 0 of any swp_type other than 0.

Note that fork's copy_page_range() does not "copy ptes where a page
fault will fill them correctly", so would in effect put a pte_none
into the child where the parent has this uffd_wp entry.  I don't know
anything about uffd versus fork, whether that would pose a problem.

Hugh