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

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Message-ID: <20171219124646.GR2787@dhcp22.suse.cz>
Date:   Tue, 19 Dec 2017 13:46:46 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Edward Napierala' <trasz@...ebsd.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-api@...r.kernel.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" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arch@...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" <jasone@...gle.com>,
        "davidtgoldblatt@...il.com" <davidtgoldblatt@...il.com>
Subject: Re: [PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE

On Tue 19-12-17 12:40:16, David Laight wrote:
> From: Edward Napierala
> > Sent: 14 December 2017 14:55
> >
> > 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.
> 
> I don't remember any discussion about using MAP_FIXED | MAP_EXCL ?
> 
> Why not match the prior art??

See the cover letter which explains why an extension is not backward
compatible.
-- 
Michal Hocko
SUSE Labs