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Date:   Thu, 03 Jun 2021 14:48:11 +0000
To:     Matthew Wilcox <willy@infradead.org>
From:   Simon Ser <contact@emersion.fr>
Cc:     Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Lin, Ming" <minggr@gmail.com>, Peter Xu <peterx@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Dan Williams <dan.j.williams@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Herrmann <dh.herrmann@gmail.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Greg Kroah-Hartman <greg@kroah.com>,
        "tytso@mit.edu" <tytso@mit.edu>
Reply-To: Simon Ser <contact@emersion.fr>
Subject: Re: Sealed memfd & no-fault mmap
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On Thursday, June 3rd, 2021 at 3:57 PM, Matthew Wilcox <willy@infradead.org=
> wrote:

> How about this:
>
>  - The client sets up a 1KiB shm file and sends it to the compositor.
>  - The client expands the file to 5KiB
>  - The compositor sees the new data up to 4KiB but zeroes past the 4KiB
>    mark.
>
> Does that still make userspace happy?

As long as the new data in the expanded region is visible after a remapping=
 the
file with the new size, it should be fine. It doesn't matter that it's not
visible without a munmap+mmap cycle.