linux-kernel - Re: [patch] mremap.2: Add description of old

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Message-ID: <20170925123508.pzjbe7wgwagnr5li@dhcp22.suse.cz>
Date:   Mon, 25 Sep 2017 14:35:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, mtk.manpages@...il.com,
        linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-mm@...ck.org
Subject: Re: [patch] mremap.2: Add description of old_size == 0 functionality

On Tue 19-09-17 14:11:19, Florian Weimer wrote:
> On 09/18/2017 07:11 PM, Mike Kravetz wrote:
[...]
> > I can drop this wording, but would still like to suggest memfd_create as
> > the preferred method of creating duplicate mappings.  It would be good if
> > others on Cc: could comment as well.
> 
> mremap seems to work with non-anonymous mappings, too:

only for shared mappings in fact. Because once we have CoW then mremap
will not provide you with the same content as the original mapping.

[...]

> > Just curious, does glibc make use of this today?  Or, is this just something
> > that you think may be useful.
> 
> To my knowledge, we do not use this today.  But it certainly looks very
> useful.

What would be the usecase. I mean why don't you simply create a new
mapping by a plain mmap when you have no guarantee about the same
content?
-- 
Michal Hocko
SUSE Labs