Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

From: Pavel Machek <pavel@ucw.cz>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Matthew Wilcox <willy@infradead.org>,
	Kees Cook <keescook@chromium.org>, Cyril Hrubis <chrubis@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Khalid Aziz <khalid.aziz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Florian Weimer <fweimer@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
	Joel Stanley <joel@jms.id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
Date: Fri, 8 Dec 2017 15:27:14 +0100	[thread overview]
Message-ID: <20171208142714.GB7793@amd> (raw)
In-Reply-To: <87shclh3zc.fsf@concordia.ellerman.id.au>

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On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> 
> > On Thu, Dec 07, 2017 at 11:14:27AM -0800, Kees Cook wrote:
> >> On Wed, Dec 6, 2017 at 9:46 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> >> > Matthew Wilcox <willy@infradead.org> writes:
> >> >> So, just like we currently say "exactly one of MAP_SHARED or MAP_PRIVATE",
> >> >> we could add a new paragraph saying "at most one of MAP_FIXED or
> >> >> MAP_REQUIRED" and "any of the following values".
> >> >
> >> > MAP_REQUIRED doesn't immediately grab me, but I don't actively dislike
> >> > it either :)
> >> >
> >> > What about MAP_AT_ADDR ?
> >> >
> >> > It's short, and says what it does on the tin. The first argument to mmap
> >> > is actually called "addr" too.
> >> 
> >> "FIXED" is supposed to do this too.
> >> 
> >> Pavel suggested:
> >> 
> >> MAP_ADD_FIXED
> >> 
> >> (which is different from "use fixed", and describes why it would fail:
> >> can't add since it already exists.)
> >> 
> >> Perhaps "MAP_FIXED_NEW"?
> >> 
> >> There has been a request to drop "FIXED" from the name, so these:
> >> 
> >> MAP_FIXED_NOCLOBBER
> >> MAP_FIXED_NOREPLACE
> >> MAP_FIXED_ADD
> >> MAP_FIXED_NEW
> >> 
> >> Could be:
> >> 
> >> MAP_NOCLOBBER
> >> MAP_NOREPLACE
> >> MAP_ADD
> >> MAP_NEW
> >> 
> >> and we still have the unloved, but acceptable:
> >> 
> >> MAP_REQUIRED
> >> 
> >> My vote is still for "NOREPLACE" or "NOCLOBBER" since it's very
> >> specific, though "NEW" is pretty clear too.
> >
> > How about MAP_NOFORCE?
> 
> It doesn't tell me that addr is not a hint. That's a crucial detail.
> 
> Without MAP_FIXED mmap never "forces/replaces/clobbers", so why would I
> need MAP_NOFORCE if I don't have MAP_FIXED?
> 
> So it needs something in there to indicate that the addr is not a hint,
> that's the only thing that flag actually *does*.
> 
> 
> If we had a time machine, the right set of flags would be:
> 
>   - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>   - MAP_REPLACE: replace an existing mapping (or force or clobber)

Actually, if we had a time machine... would we even provide
MAP_REPLACE functionality?

> But the two were conflated for some reason in the current MAP_FIXED.
> 
> Given we can't go back and fix it, the closest we can get is to add a
> variant of MAP_FIXED which subtracts the "REPLACE" semantic.
> 
> ie: MAP_FIXED_NOREPLACE

I like MAP_FIXED_NOREPLACE.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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