linux-kernel - Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE

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Date:   Fri, 9 Nov 2018 12:36:34 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, jreck@...gle.com,
        john.stultz@...aro.org, tkjos@...gle.com,
        gregkh@...uxfoundation.org, hch@...radead.org,
        Al Viro <viro@...iv.linux.org.uk>, dancol@...gle.com,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Lei Yang <Lei.Yang@...driver.com>,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org,
        Marc-André Lureau 
        <marcandre.lureau@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, minchan@...nel.org,
        Shuah Khan <shuah@...nel.org>, valdis.kletnieks@...edu
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to
 memfd

On Wed,  7 Nov 2018 20:15:36 -0800 "Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:

> Android uses ashmem for sharing memory regions. We are looking forward
> to migrating all usecases of ashmem to memfd so that we can possibly
> remove the ashmem driver in the future from staging while also
> benefiting from using memfd and contributing to it. Note staging drivers
> are also not ABI and generally can be removed at anytime.
> 
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then add protection against making any
> "future" writes while keeping the existing already mmap'ed
> writeable-region active.  This allows us to implement a usecase where
> receivers of the shared memory buffer can get a read-only view, while
> the sender continues to write to the buffer.
> See CursorWindow documentation in Android for more details:
> https://developer.android.com/reference/android/database/CursorWindow

It appears that the memfd_create and fcntl manpages will require
updating.  Please attend to this at the appropriate time?

Actually, it would help the review process if those updates were
available now.