linux-kernel - Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags

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Date:   Tue, 15 Aug 2017 15:31:04 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding
 new mmap flags

On Tue, Aug 15, 2017 at 9:28 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Mon, Aug 14, 2017 at 11:12 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> The mmap syscall suffers from the ABI anti-pattern of not validating
>> unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
>> mechanism to define new behavior that is known to fail on older kernels
>> without the feature. Use the fact that specifying MAP_SHARED and
>> MAP_PRIVATE at the same time is invalid as a cute hack to allow a new
>> set of validated flags to be introduced.
>
> While this is cute, is it actually better than a new syscall?

After playing with MAP_DIRECT defined as (MAP_SHARED|MAP_PRIVATE|0x40)
I think a new syscall is better. It's very easy to make the mistake
that "MAP_DIRECT" defines a single flag vs representing a multi-bit
encoding.