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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 31 May 2021 20:24:07 -1000
Message-ID: <CAHk-=whce3vmj+g7jcE0rasoDavJutxno3ZZrvvWYQywWXH31Q@mail.gmail.com>
Subject: Re: Sealed memfd & no-fault mmap
To:     Ming Lin <minggr@gmail.com>
Cc:     Hugh Dickins <hughd@google.com>, Simon Ser <contact@emersion.fr>,
        Peter Xu <peterx@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        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>
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On Mon, May 31, 2021 at 11:13 AM Ming Lin <minggr@gmail.com> wrote:
>
> OK, I borrowed code from filemap_xip.c and implemented this behavior.

I think that "unmap page" is too complicated and fragile.

The only page that could possibly validly be unmapped is a stale zero
page, but that code in shmem_unmap_nofault_page() seems to try to
handle other cases too (ie that whole page_remove_rmap() - afaik a
zero page has no rmap).

I get the feeling that the simpler thing to do is to just say "if you
use MAP_NOSIGBUS, and you access pages that don't have a backing
store, you will get zero pages, and they will NOT BE SYNCHRONIZED with
the backing store possibly later being updated".

IOW, just document the fact that a MAP_NOSIGBUS mapping isn't coherent
wrt shmem contents that are expanded and filled in later.

Don't try to "fix" it - because any user that uses MAP_NOSIGBUS had
better just accept that it's not compatible with expanding the shmem
backing store later.

Keep it simple and stupid. Hmm?

                  Linus