linux-kernel - Re: Sealed memfd & no-fault mmap

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Message-ID: <yK5pBcI8tfOfgvVkLrLRMaAdSrt7uL8Ppc05GLMMl8JEfBVsY7f-ksJME4RsNJUvOif2u_r_dBh_PwXyPwYl4Ug2tiMiMxVJb7ESHKNS6w4=@emersion.fr>
Date:   Thu, 03 Jun 2021 14:48:11 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Lin, Ming" <minggr@...il.com>, Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Herrmann <dh.herrmann@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <greg@...ah.com>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: Sealed memfd & no-fault mmap

On Thursday, June 3rd, 2021 at 3:57 PM, Matthew Wilcox <willy@...radead.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.