linux-kernel - Re: [PATCH v2] mmap.2: document new MAP_FIXED

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Date:   Thu, 30 Nov 2017 19:39:14 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        linux-api@...r.kernel.org, Khalid Aziz <khalid.aziz@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>
Subject: Re: [PATCH v2] mmap.2: document new MAP_FIXED_SAFE flag

On Thu 30-11-17 10:31:12, John Hubbard wrote:
> On 11/30/2017 12:24 AM, Michal Hocko wrote:
> > Updated version based on feedback from John.
> > ---
> > From ade1eba229b558431581448e7d7838f0e1fe2c49 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@...e.com>
> > Date: Wed, 29 Nov 2017 15:32:08 +0100
> > Subject: [PATCH] mmap.2: document new MAP_FIXED_SAFE flag
> > 
> > 4.16+ kernels offer a new MAP_FIXED_SAFE flag which allows the caller to
> > atomicaly probe for a given address range.
> > 
> > [wording heavily updated by John Hubbard <jhubbard@...dia.com>]
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> > ---
> >  man2/mmap.2 | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/man2/mmap.2 b/man2/mmap.2
> > index 385f3bfd5393..923bbb290875 100644
> > --- a/man2/mmap.2
> > +++ b/man2/mmap.2
> > @@ -225,6 +225,22 @@ will fail.
> >  Because requiring a fixed address for a mapping is less portable,
> >  the use of this option is discouraged.
> >  .TP
> > +.BR MAP_FIXED_SAFE " (since Linux 4.16)"
> > +Similar to MAP_FIXED with respect to the
> > +.I
> > +addr
> > +enforcement, but different in that MAP_FIXED_SAFE never clobbers a pre-existing
> > +mapped range. If the requested range would collide with an existing
> > +mapping, then this call fails with
> > +.B EEXIST.
> > +This flag can therefore be used as a way to atomically (with respect to other
> > +threads) attempt to map an address range: one thread will succeed; all others
> > +will report failure. Please note that older kernels which do not recognize this
> > +flag will typically (upon detecting a collision with a pre-existing mapping)
> > +fall back a "non-MAP_FIXED" type of behavior: they will return an address that
> 
> ...and now I've created my own typo: please make that "fall back to a"  (the 
> "to" was missing).
> 
> Sorry about the churn. It turns out that the compiler doesn't catch these. :)

Fixed. I will resubmit after there is more feedback review.

Thanks
-- 
Michal Hocko
SUSE Labs