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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 12 Oct 2017 09:32:17 -0700
Message-ID: <CA+55aFyy-nz99c6erFh=aeyCOzsk0td5wHaVLpwBNA-sWNDZkA@mail.gmail.com>
Subject: Re: [PATCH v9 1/6] mm: introduce MAP_SHARED_VALIDATE, a mechanism to
 safely define new mmap flags
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Sender: owner-linux-mm@kvack.org
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To: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, Arnd Bergmann <arnd@arndb.de>, Linux API <linux-api@vger.kernel.org>, linux-xfs <linux-xfs@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andy Lutomirski <luto@kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Christoph Hellwig <hch@lst.de>

On Thu, Oct 12, 2017 at 6:51 AM, Jan Kara <jack@suse.cz> wrote:
>
> When thinking a bit more about this I've realized one problem: Currently
> user can call mmap() with MAP_SHARED type and MAP_SYNC or MAP_DIRECT flags
> and he will get the new semantics (if the kernel happens to support it).  I
> think that is undesirable [..]

Why?

If you have a performance preference for MAP_DIRECT or something like
that, but you don't want to *enforce* it, you'd use just plain
MAP_SHARED with it.

Ie there may well be "I want this to work, possibly with downsides" issues.

So it seems to be a reasonable model, and disallowing it seems to
limit people and not really help anything.

                 Linus

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