linux-kernel - Re: [PATCH v2 0/2] mm: introduce MAP_FIXED

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Message-ID: <20171214145443.GA2202@brick>
Date:   Thu, 14 Dec 2017 14:54:43 +0000
From:   Edward Napierala <trasz@...ebsd.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-api@...r.kernel.org, Khalid Aziz <khalid.aziz@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
        Florian Weimer <fweimer@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>,
        Kees Cook <keescook@...omium.org>, jasone@...gle.com,
        davidtgoldblatt@...il.com
Subject: Re: [PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE

On 1214T1415, Michal Hocko wrote:
> On Thu 14-12-17 12:44:17, Edward Napierala wrote:
> > Regarding the name - how about adopting MAP_EXCL?  It was introduced in
> > FreeBSD,
> > and seems to do exactly this; quoting mmap(2):
> > 
> > MAP_FIXED    Do not permit the system to select a different address
> >                         than the one specified.  If the specified address
> >                         cannot be used, mmap() will fail.  If MAP_FIXED is
> >                         specified, addr must be a multiple of the page size.
> >                         If MAP_EXCL is not specified, a successful MAP_FIXED
> >                         request replaces any previous mappings for the
> >                         process' pages in the range from addr to addr + len.
> >                         In contrast, if MAP_EXCL is specified, the request
> >                         will fail if a mapping already exists within the
> >                         range.
> 
> I am not familiar with the FreeBSD implementation but from the above it
> looks like MAP_EXCL is a MAP_FIXED mofifier which is not how we are
> going to implement it in linux due to reasons mentioned in this cover
> letter. Using the same name would be more confusing than helpful I am
> afraid.

Sorry, missed that.  Indeed, reusing a name with a different semantics
would be a bad idea.